Publications by authors named "John A Howarter"

23 Publications

  • Page 1 of 1

Structure-Property Relationship of Cellulose Nanocrystal-Polyvinyl Alcohol Thin Films for High Barrier Coating Applications.

ACS Appl Mater Interfaces 2021 Mar 3. Epub 2021 Mar 3.

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States.

CO and O gas permeability are paramount concerns in food packaging. Here, the permeability of cellulose nanocrystals (CNCs) and polyvinyl alcohol (PVA) coatings was explored as it relates to varied CNC content. Specifically, this work focuses on the role of PVA in rheology and barrier performance of the CNC films. Results show that shear-casted CNC films are transparent and have a high-order parameter, which is attributed to the shear-thinning behavior of the CNCs. The barrier performance of the CNC films improved because of the synergistic effect of having both alignment of CNCs and a lower free volume. The CNC-PVA films exhibited excellent barrier performance as compared to traditional engineered polymers, even much higher than high barrier ethylene-vinyl alcohol copolymer films. Furthermore, the moisture sensitivity of the films was greatly diminished with the addition of PVA. Overall, the results show applicability of CNC-PVA coating formulations for high barrier packaging applications.
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http://dx.doi.org/10.1021/acsami.0c21525DOI Listing
March 2021

Reactivity of the Polyamide Membrane Monomer with Free Chlorine: Role of Bromide.

Environ Sci Technol 2021 Feb 26;55(4):2575-2584. Epub 2021 Jan 26.

Lyles School of Civil Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907, United States.

Aromatic polyamide-based membranes are widely used for reverse osmosis (RO) and nanofiltration (NF) treatment but degrade when exposed to free chlorine (HOCl/OCl). The reaction mechanisms with free chlorine were previously explored, but less is known about the role of bromide (Br) in these processes. Br may impact these reactions by reacting with HOCl to form HOBr, which then triggers other brominating agents (BrO, Br, BrOCl, and BrCl) to form. This study examined the reactivities of these brominating agents with a polyamide monomer model compound, benzanilide (BA), and a modified version of it, -CH-BA. The results indicated that all these brominating agents only attacked the aromatic ring adjacent to the amide N, rather than the amide N, different from the previously examined chlorinating agents (HOCl, OCl, and Cl) that attacked both sites. Orton rearrangement was not observed. Species-specific rate constants (, M s) between BA and HOBr, BrO, Br, BrOCl, and BrCl were determined to be (5.3 ± 1.2) × 10, (1.2 ± 0.4) × 10, (3.7 ± 0.2) × 10, (2.2 ± 0.6) × 10, and (6.6 ± 0.9) × 10 M s, respectively, such that > > > > . -CH-BA exhibited lower reactivity than BA. Model predictions of BA loss during chlorination with varied Br and/or Cl concentrations were established. These findings will ultimately enable membrane degradation and performance loss following chlorination in mixed halide solutions to be better predicted during pilot- and full-scale NF and RO treatment.
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http://dx.doi.org/10.1021/acs.est.0c06122DOI Listing
February 2021

An emerging mobile air pollution source: outdoor plastic liner manufacturing sites discharge VOCs into urban and rural areas.

Environ Sci Process Impacts 2020 Sep;22(9):1828-1841

Lyles School of Civil Engineering, Division of Environmental & Ecological Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana, USA 47907.

The in situ manufacture of cured-in-place-pipe (CIPP) plastic liners in damaged sewer pipes is an emerging mobile source of anthropogenic air pollution. Evidence indicates volatile organic compounds (VOCs) can be released before, during, and after manufacture. The chemical composition of a popular uncured styrene-based CIPP resin was examined, along with the VOCs that remained in the new cured composite. The roles of curing temperature and heating time in waste discharged into the air were examined. Uncured resin contained approximately 39 wt% VOCs. Multiple hazardous air pollutants were present, however, 61 wt% of the uncured resin was not chemically identified. A substantial mass of VOCs (8.87 wt%) was emitted into the air during manufacture, and all cured composites contained about 3 wt% VOCs. Some VOCs were created during manufacture. Curing temperature (65.5-93.3 °C) and heating time (25-100 min) did not cause different composite VOC loadings. High styrene air concentrations inhibited the detection of other VOCs in air. It is estimated that tens of tons of VOCs may be emitted at a single CIPP manufacturing site. Regulators should consider monitoring, and potentially regulating, these growing mobile air pollution and volatile chemical product sources as they are operating in urban and rural areas often in close proximity to residential and commercial buildings.
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http://dx.doi.org/10.1039/d0em00190bDOI Listing
September 2020

Diffusion-Controlled Spontaneous Emulsification of Water-Soluble Oils via Micelle Swelling.

Langmuir 2020 Jul 24;36(26):7517-7527. Epub 2020 Jun 24.

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States.

Spontaneous emulsification of toluene, xylenes, cyclohexane, and mineral oil in a nonionic nonylphenol polyethoxylate surfactant solution was investigated by visual observations coupled with dynamic light scatting measurements and interfacial tensiometry. For water-soluble oils, nanoscale emulsions formed spontaneously by diffusion of oil molecules into the aqueous surfactant solutions and subsequent swelling of surfactant micelles with oil. Micelle swelling rates were quantified to assess system spontaneity, revealing that oil solubility in water was directly correlated to the spontaneity of the emulsion (toluene > xylenes > cyclohexane). When experiments were intentionally designed to create surfactant concentration gradients, Marangoni flows were found to enhance spontaneity. Despite their spontaneous formation, emulsion stability was limited over the course of 40 days by Ostwald ripening followed by creaming and evaporation. These results provide insights on the likelihood of nanoemulsion formation and persistence in oily wastewater as the components in this study are present in many wastewater systems.
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http://dx.doi.org/10.1021/acs.langmuir.0c01121DOI Listing
July 2020

Influence of Free Volume Determined by Positron Annihilation Lifetime Spectroscopy (PALS) on Gas Permeability of Cellulose Nanocrystal Films.

ACS Appl Mater Interfaces 2020 May 15;12(21):24380-24389. Epub 2020 May 15.

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States.

Cellulose nanocrystals (CNCs) are of increasing interest for packaging applications because of their biodegradability, low cost, high crystallinity, and high aspect ratio. The objective of this study was to use positron annihilation lifetime spectroscopy (PALS) to investigate the free volume of CNC films with different structural arrangements (chiral nematic vs shear-oriented CNC films) and relate this information to gas barrier performance. It was found that sheared CNC films with higher CNC alignment have lower free volume and hence have more tortuosity than chiral nematic self-assembled films, which lowers gas diffusion throughout the films. The overall barrier performance of the aligned CNC film obtained in this study has a higher barrier performance than high barrier polymer films like PVOH and EVOH. Furthermore, a modified model was developed for single-component CNC films to predict the gas permeability with variation of CNC alignment with validation by the data taken.
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http://dx.doi.org/10.1021/acsami.0c05738DOI Listing
May 2020

Reactivity of the Polyamide Membrane Monomer with Free Chlorine: Reaction Kinetics, Mechanisms, and the Role of Chloride.

Environ Sci Technol 2019 Jul 20;53(14):8167-8176. Epub 2019 Jun 20.

Lyles School of Civil Engineering , Purdue University , 550 Stadium Mall Drive , West Lafayette , Indiana 47907 , United States.

Aromatic polyamide thin-film composite membranes are widely used in reverse osmosis (RO) and nanofiltration (NF) due to their high water permeability and selectivity. However, these membranes undergo biofouling and can degrade and eventually fail during free chlorine exposure. To better understand this effect, the reactivity of the polyamide monomer (benzanilide (BA)) with free chlorine was tested under varying pH and chloride (Cl) conditions. The kinetic results indicated that the current existing mechanisms, especially the Orton rearrangement, were invalid. Revised reaction pathways were proposed where BA chlorination was driven by two independent pathways involving the anilide ring and amide nitrogen moieties. The ability for one moiety to be chosen over the other was highly dependent on the pH, Cl concentration, and the resulting chlorinating agents (e.g., Cl, HOCl, OCl, and ClO) generated. Species-specific rate constants for BA with Cl, OCl, and HOCl equaled (7.6 ± 0.19) × 10, (1.7 ± 1.5) × 10, (2.1 ± 0.71) × 10 M s, respectively. A similar value for ClO could not be accurately estimated under the tested conditions. The behavior of these chlorinating agents differed for each reactive site such that OCl > HOCl for -chlorination and Cl > HOCl > OCl for anilide ring chlorination. Experiments with modified monomers indicated that substituent placement largely affected which reactive site was kinetically favorable. Overall, such findings provide a predictive model of how the polyamide monomer degrades during chlorine exposure and guidance on how chlorine-resistant polyamide membranes should be designed.
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http://dx.doi.org/10.1021/acs.est.9b01446DOI Listing
July 2019

toxicity assessment of emitted materials collected during the manufacture of water pipe plastic linings.

Inhal Toxicol 2019 03 12;31(4):131-146. Epub 2019 Jun 12.

a School of Health Sciences, College of Human and Health Sciences , Purdue University , West Lafayette , IN , USA.

US water infrastructure is in need of widespread repair due to age-related deterioration. Currently, the cured-in-place (CIPP) procedure is the most common method for water pipe repair. This method involves the on-site manufacture of a new polymer composite plastic liner within the damaged pipe. The CIPP process can release materials resulting in occupational and public health concerns. To understand hazards associated with CIPP-related emission exposures, an toxicity assessment was performed. Mouse alveolar epithelial and alveolar macrophage cell lines and condensates collected at 3 worksites utilizing styrene-based resins were utilized for evaluations. All condensate samples were normalized based on the major emission component, styrene. Further, a styrene-only exposure group was used as a control to determine mixture related toxicity. Cytotoxicity differences were observed between worksite samples, with the CIPP worksite 4 sample inducing the most cell death. A proteomic evaluation was performed, which demonstrated styrene-, worksite-, and cell-specific alterations. This examination of protein expression changes determined potential biomarkers of exposure including transglutaminase 2, advillin, collagen type 1, perilipin-2, and others. Pathway analysis of exposure-induced proteomic alterations identified MYC and p53 to be regulators of cellular responses. Protein changes were also related to pathways involved in cell damage, immune response, and cancer. Together these findings demonstrate potential risks associated with the CIPP procedure as well as variations between worksites regarding emissions and toxicity. Our evaluation identified biological pathways that require a future evaluation and also demonstrates that exposure assessment of CIPP worksites should examine multiple chemical components beyond styrene, as many cellular responses were styrene-independent.
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http://dx.doi.org/10.1080/08958378.2019.1621966DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6639800PMC
March 2019

Considerations for emission monitoring and liner analysis of thermally manufactured sewer cured-in-place-pipes (CIPP).

J Hazard Mater 2019 06 27;371:540-549. Epub 2019 Feb 27.

Division of Ecological and Environmental Engineering, 500 Central Drive, Purdue University, West Lafayette, 47907, IN, USA; Lyles School of Civil Engineering, 550 Stadium Mall Drive, Purdue University, West Lafayette, 47907, IN, USA. Electronic address:

Cured-in-place-pipes (CIPP) are plastic liners chemically manufactured inside existing damaged sewer pipes. They are gaining popularity in North America, Africa, Asia, Europe, and Oceania. Volatile and semi-volatile organic compound (VOC/SVOC) emissions from storm sewer CIPP installations were investigated at a dedicated outdoor research site. Tedlar bag, sorbent tube, and photoionization detector (PID) air sampling was conducted for five steam-CIPP installations and was coupled with composite characterizations. New CIPPs contained up to 2.21 wt% volatile material and only 6-31% chemical mass extracted per CIPP was identified. Each 6.1 m [20 ft] liner contained an estimated 5-10 kg [11-22 lbs] of residual chemical. Extracted chemicals included hazardous air pollutants and suspected and known carcinogens that were not reported by others. These included monomers, monomer oxidation products, antioxidants, initiator degradation products, and a plasticizer. PID signals did not accurately represent styrene air concentration differing sometimes by 10s- to 1000s-fold. Multiple VOCs found in air samples likely affected PID responses. Styrene (>86.4 ppm) and methylene chloride (>1.56 ppm) air concentrations were likely greater onsite and phenol was also detected. Additional studies are needed to examine pollutant emissions so process monitoring can be improved, and environment impacts and associated human exposures can be minimized.
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http://dx.doi.org/10.1016/j.jhazmat.2019.02.097DOI Listing
June 2019

Assessment of early onset surface damage from accelerated disinfection protocol.

Antimicrob Resist Infect Control 2019 31;8:24. Epub 2019 Jan 31.

1School of Materials Engineering, Purdue University, 701 W. Stadium Avenue, West Lafayette, IN 47907 USA.

Background: The objective of this study was to evaluate the extent and potential mechanisms of early onset surface damage from simulated wiping typical of six-months of routine disinfection and to assess the subsequent microbial risk of surfaces damaged by disinfectants.

Methods: Eight common material surfaces were exposed to three disinfectants and a neutral cleaner (neutral cleaner, quaternary ammonium, hydrogen peroxide, sodium hypochlorite) in accelerated aging tests to simulate a long-term disinfection routine. Materials were also immersed in dilute and concentrated chemical solutions to induce surface damage. Surfaces were chemically and physically characterized to determine extent of surface damage. Bactericidal efficacy testing was performed on the Quat-based disinfectant using a modified version of EPA standard operating procedure MB-25-02.

Results: The wiping protocol increased surface roughness for some material surfaces due to mechanical abrasion of the wiping cloth. The increased roughness did not correlate with changes in bactericidal efficacy. Chemical damage was observed for some surface-disinfectant combinations. The greatest observed effects from disinfectant exposure was in changes in wettability or water contact angle.

Conclusions: Early onset surface damage was observed in chemical and physical characterization methods. These high-throughput material measurement methods were effective at assessing nanoscale disinfectant-surface compatibility which may go undetected though routine macroscale testing.
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http://dx.doi.org/10.1186/s13756-019-0467-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6357435PMC
March 2020

Outdoor manufacture of UV-Cured plastic linings for storm water culvert repair: Chemical emissions and residual.

Environ Pollut 2019 Feb 30;245:1031-1040. Epub 2018 Oct 30.

Lyles School of Civil Engineering and Division of Ecological and Environmental Engineering, Purdue University, West Lafayette, IN, 47907, USA. Electronic address:

Storm water culverts are integral for U.S. public safety and welfare, and their mechanical failure can cause roadways to collapse. To repair these buried assets, ultraviolet (UV) light cured-in-place-pipes (CIPP) are being installed. Chemical emission and residual material left behind from the installation process was investigated in New York and Virginia, USA. Samples of an uncured resin tube and field-cured styrene-based resin CIPPs were collected and analyzed. Also collected were air and water samples before, during, and after installations. Chemicals were emitted into air because of the installation and curing processes. Particulates emitted into the air, water, and soil contained fiberglass, polymer, and contaminants, some of which are regulated by state-level water quality standards. The uncured resin tube contained more than 70 chemical compounds, and 19 were confirmed with analytical standards. Compounds included known and suspected carcinogens, endocrine disrupting compounds, hazardous air pollutants, and other compounds with little aquatic toxicity data available. Compounds (14 of 19 confirmed) were extracted from the newly installed CIPPs, and 11 were found in water samples. Aqueous styrene (2.31 mg/L), dibutyl phthalate (12.5 μg/L), and phenol (16.7 μg/L) levels exceeded the most stringent state water quality standards chosen in this study. Styrene was the only compound that was found to have exceed a 48 h aquatic toxicity threshold. Newly installed CIPPs contained a significant amount volatile material (1.0 to > 9.0 wt%). Recommendations provided can reduce chemical emission, as well as improve worksite and environmental protection practices. Recommended future research is also described.
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http://dx.doi.org/10.1016/j.envpol.2018.10.080DOI Listing
February 2019

Investigation of the factors that influence lead accumulation onto polyethylene: Implication for potable water plumbing pipes.

J Hazard Mater 2018 04 29;347:242-251. Epub 2017 Dec 29.

Division of Environmental and Ecological Engineering, Lyles School of Civil Engineering, 550 Stadium Mall Drive West Lafayette, IN, 47907, USA. Electronic address:

The influence of polymer aging, water pH, and aqueous Pb concentration on Pb deposition onto low density polyethylene (LDPE) was investigated. LDPE pellets were aged by ozonation at 85 °C. ATR-FTIR and X-ray photoelectron spectroscopy (XPS) analysis of aged LDPE surfaces showed that a variety of polar functional groups (>CO<, >CO, >COO) were formed during aging. These functional groups likely provided better nucleation sites for Pb(OH) deposition compared to new LDPE, which did not have these oxygen-containing functional groups. The type and amount of Pb species present on these surfaces were evaluated through XPS. The influence of exposure duration on Pb deposition onto LDPE was modeled using the pseudo-first-order equation. Distribution ratios of 251.5 for aged LDPE and 69.3 for new LDPE showed that Pb precipitates had greater affinity for the surface of aged LDPE compared to new LDPE. Aged LDPE had less Pb surface loading at pH 11 compared to loading at pH 7.8. Pb surface loading for aged LDPE changed linearly with aging duration (from 0.5-7.5 h). Pb surface loading on both new and aged LDPE increased linearly with increasing Pb initial concentration. Greater Pb precipitation rates were found for aged LDPE compared to new LDPE at both tested pH values.
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http://dx.doi.org/10.1016/j.jhazmat.2017.12.066DOI Listing
April 2018

The interaction of surfactants with plastic and copper plumbing materials during decontamination.

J Hazard Mater 2017 Mar 25;325:8-16. Epub 2016 Nov 25.

Division of Environmental and Ecological Engineering and Lyles School of Civil Engineering, 550 Stadium Mall Drive, Purdue University, West Lafayette, IN 47907, USA. Electronic address:

The study goal was to examine the effectiveness of surfactants to decontaminate plastic and copper potable water plumbing components. Several common potable water pipe and gasket plastics were examined as well as Alconox detergent, Dawn soap, and MAGIT-DG 100 surfactants. Results showed that the MAGIT-DG 100 solutions permeated all plastics within 3days, effectively compromising tensile strength (-82%), physical dimension (+43% volume, +45% weight), and oxidative resistance (-15%). A variety of MAGIT-DG 100 solution compounds permeated plastic samples, not just the declared major ingredient. PVC and cPVC pipes sorbed the least amount of this solution's components of all the plastic pipes tested. Alconox and Dawn solutions caused minimal changes to the physical and mechanical properties of all plastics examined. Crosslinked polyethylene type A (PEX-a) pipe was more susceptible to crude oil contamination than copper pipe. Flushing with a pure water Alconox solution mixture removed all benzene, toluene, ethylbenzene, and total xylenes (BTEX) from copper pipe. No decontamination method affected BTEX removal from PEX pipe. Under certain conditions surfactant solutions have the potential to alter material integrity and may not be a viable option in removing hydrophobic organic compounds from plastic pipe.
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http://dx.doi.org/10.1016/j.jhazmat.2016.11.067DOI Listing
March 2017

Hydrothermal Synthesis and Processing of Barium Titanate Nanoparticles Embedded in Polymer Films.

ACS Appl Mater Interfaces 2015 Dec 15;7(51):28640-6. Epub 2015 Dec 15.

School of Materials Engineering, Purdue University , 701 W. Stadium Avenue, West Lafayette, Indiana 47907, United States.

Barium titanate nanoparticles embedded in flexible polymer films were synthesized using hydrothermal processing methods. The resulting films were characterized with respect to material composition, size distribution of nanoparticles, and spatial location of particles within the polymer film. Synthesis conditions were varied based on the mechanical properties of the polymer films, ratio of polymer to barium titanate precursors, and length of aging time between initial formulations of the solution to final processing of nanoparticles. Block copolymers of poly(styrene-co-maleic anhydride) (SMAh) were used to spatially separate titanium precursors based on specific chemical interactions with the maleic anhydride moiety. However, the glassy nature of this copolymer restricted mobility of the titanium precursors during hydrothermal processing. The addition of rubbery butadiene moieties, through mixing of the SMAh with poly(styrene-butadiene-styrene) (SBS) copolymer, increased the nanoparticle dispersion as a result of greater diffusivity of the titanium precursor via higher mobility of the polymer matrix. Additionally, an aminosilane was used as a means to retard cross-linking in polymer-metalorganic solutions, as the titanium precursor molecules were shown to react and form networks prior to hydrothermal processing. By adding small amounts of competing aminosilane, excessive cross-linking was prevented without significantly impacting the quality and composition of the final barium titanate nanoparticles. X-ray diffraction and X-ray photoelectron spectroscopy were used to verify nanoparticle compositions. Particle sizes within the polymer films were measured to be 108 ± 5 nm, 100 ± 6 nm, and 60 ± 5 nm under different synthetic conditions using electron microscopy. Flexibility of the films was assessed through measurement of the glass transition temperature using dynamic mechanical analysis. Dielectric permittivity was measured using an impedance analyzer.
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http://dx.doi.org/10.1021/acsami.5b10282DOI Listing
December 2015

Enhanced Wettability and Transport Control of Ultrafiltration and Reverse Osmosis Membranes with Grafted Polyelectrolytes.

ACS Appl Mater Interfaces 2015 Nov 29;7(44):24839-47. Epub 2015 Oct 29.

School of Materials Engineering, Purdue University , 701 West Stadium Avenue, West Lafayette, Indiana 47907, United States.

End-functionalized poly(acrylic acid) (PAA-silane) was synthesized with reversible addition-fragmentation chain-transfer (RAFT) polymerization and attached to both polysulfone ultrafiltration (UF) and polyamide reverse osmosis (RO) membranes through a nonimpairing, one-step grafting to approach in order to improve membrane surface wettability with minimal impact on membrane transport performance. After PAA grafting, composition and morphology changes on the membrane surface were characterized with Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). Static contact angle on PAA grafted membranes exhibited an increase in surface hydrophilicity and hence a potential enhancement in antifouling performance. The native contact angle on the polysulfone membrane systems was 86° and was reduced to 24° after modification, while the polyamide film contact angle decreased from 58° to 25°. The PAA layer endowed the porous UF membrane with dynamic control over the permeability and selectivity through the manipulation of the solution pH. The UF membrane with a 35 nm average pore size displayed a 115% increase in flux when the contact solution was changed from pH 11 to pH 3. This effect was diminished to 70% and 32% as the average pore size decreased to 20 and 10 nm, respectively. Modified RO membranes displayed no reduction in membrane performance indicating that the underlying materials were unaffected by the modification environment or added polymer. Model polyamide and polysulfone surfaces were reacted with the PAA-silane inside a quartz crystal microbalance (QCM) to help inform the deposition behavior for the respective membrane chemistries.
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http://dx.doi.org/10.1021/acsami.5b08046DOI Listing
November 2015

QCM-based measurement of chlorine-induced polymer degradation kinetics.

Langmuir 2014 Jul 17;30(29):8923-30. Epub 2014 Jul 17.

School of Materials Engineering, Purdue University , West Lafayette, Indiana 47907, United States.

Highly structured network polymers are prepared via a molecular layer by layer technique (mLbL) and used as a model system to study aqueous degradation of polymer thin films in real time. Quantitative analysis of the degradation kinetics was enabled by the use of a quartz crystal microbalance (QCM). We conclude that the common metric of halogen, specifically chlorine, exposure (concentration × time) to be an ineffective normalization unit and showed a multistage adsorption process consistent with the established chemical mechanism. Additionally, degradation progression was tracked at multiple points of exposure to determine the effects of chlorination on the chemical and morphological state of the polymer structure with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. The formation of known halogenation products were corroborated with XPS through the high resolution spectra. Insight into the heterogeneous nature of the nanostructural degradation was derived from the AFM images. Periodic rinsing was found to release adsorbed chlorine but had negligible benefits on extending the exposure limits of the polyamide film. Fluorinated amine monomer (3,4-difluoroaniline) was incorporated into the surface of the polymer to determine the effect of limiting N-halogenation and the formation of the halogenated ring product. The modified surface layer reduced the rate and magnitude of chlorine adsorption relative to the neat polyamide surface. The QCM technique was shown to be an effective tool for rapid and high fidelity evaluation of molecular degradation and modification strategies to increase device lifetimes.
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http://dx.doi.org/10.1021/la501922uDOI Listing
July 2014

Rational interface design of epoxy-organoclay nanocomposites: role of structure-property relationship for silane modifiers.

J Colloid Interface Sci 2014 Apr 27;419:73-8. Epub 2013 Dec 27.

School of Materials Engineering, Purdue University, 701 W Stadium Ave., West Lafayette, IN 47907, USA; Division of Environmental and Ecological Engineering, Purdue University, 500 Central Dr., West Lafayette, IN 47907, USA. Electronic address:

Montmorillonite was modified by three silane surfactants with different functionalities to investigate the role of surfactant structure on the properties of a final epoxy-organoclay nanocomposite. N-aminopropyldimethylethoxysilane (APDMES), an aminated monofunctional silane, was chosen as a promising surfactant for several reasons: (1) it will bond to silica in montmorillonite, (2) it will bond to epoxide groups, and (3) to overcome difficulties found with trifunctional aminosilane bonding clay layers together and preventing exfoliation. A trifunctional and non-aminated version of APDMES, 3-aminopropyltriethoxysilane (APTES) and n-propyldimethylmethoxysilane (PDMMS), respectively, was also studied to provide comparison to this rationally chosen surfactant. APDMES and APTES were grafted onto montmorillonite in the same amount, while PDMMS was barely grafted (<1 wt%). The gallery spacing of APDMES organoclay was greater than APTES or PDMMS, but the final nanocomposite gallery spacing was not dependent on the surfactant used. Different concentrations of APDMES modified montmorillonite yielded different properties, as concentration decreased glass transition temperature increased, thermal stability increased, and the storage modulus decreased. Storage modulus, glass transition temperature, and thermal stability were more similar for epoxy-organoclay composites modified with the same concentration of silane surfactant, neat epoxy, and epoxy-montmorillonite nanocomposite.
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http://dx.doi.org/10.1016/j.jcis.2013.12.051DOI Listing
April 2014

Effect of polymer degree of conversion on Streptococcus mutans biofilms.

Macromol Biosci 2012 Dec 5;12(12):1706-13. Epub 2012 Oct 5.

Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8543, USA.

Biofilm-material interactions are increasingly recognized as critical to success of some materials/devices and failure of others. We use a model system of dental monomers, salivary pellicles, and oral biofilms to demonstrate for the first time that degree of conversion of cross-linked dimethacrylate polymers alters biofilm metabolic activity. This response is due primarily to leachable release (not surface chemistry) and is complex, with no changes in some biofilm measurements (i.e., biomass), and time- and leachable-dependent responses in others (i.e., metabolic activity). These results highlight the need for considering biofilm-material interactions when designing/evaluating new materials.
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http://dx.doi.org/10.1002/mabi.201200214DOI Listing
December 2012

Wetting behavior of oleophobic polymer coatings synthesized from fluorosurfactant-macromers.

ACS Appl Mater Interfaces 2011 Jun 10;3(6):2022-30. Epub 2011 May 10.

School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA.

Architecturally similar monomers were copolymerized with a water-oil discriminate fluorosurfactant to create hydrophilic-oleophobic coatings. Acrylic acid, hydroxyethyl methacrylate, and methyl methacrylate were used as comonomers with the fluorosurfactant macromer. The homopolymers of the selected comonomers are water-soluble, water-swellable, and water-insoluble, respectively, thus coupling the surfactant monomer in varying concentration within polymers of varying hydrophilicity. Wetting behavior of water and hexadecane were examined as a function of copolymer composition, thus revealing critical structure-property relationships for the surfactant-based system. Acrylic acid copolymers and hydroxyethyl methacrylate copolymers both exhibited a hexadecane contact angle which exceeded the water contact angle. This condition predicted an ability to "self-clean" oil-based foulants. The most oleophobic of the self-cleaning copolymers had an advancing hexadecane contact angle of 73° and an advancing water contact angle of 40°. It was determined that the advancing and receding water and hexadecane contact angle response varies montonically for each copolymer type as the surface concentration of the surfactant is varied. Comparing between copolymer types revealed large differences in wetting response. Methyl methacrylate copolymers with 2.8 mol % surfactant had advancing water contact angle 82° and advancing hexadecane contact angle 26°, which is neither oleophobic nor self-cleaning. In contrast, acrylic acid copolymers with 3.1 mol % surfactant had advancing water contact angle of 44° and advancing hexadecane contact angle of 52°, creating a self-cleaning coating. Thus, the nature of the comonomer exerts a greater influence than the surfactant content on the wetting behavior and self-cleaning ability of the final coating.
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http://dx.doi.org/10.1021/am200255vDOI Listing
June 2011

In situ formation of silver nanoparticles in photocrosslinking polymers.

J Biomed Mater Res B Appl Biomater 2011 Apr 2;97(1):124-31. Epub 2011 Feb 2.

Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-8543, USA.

Nanocomposites of cross-linked methacrylate polymers with silver nanoparticles have been synthesized by coupling photoinitiated free radical polymerization of dimethacrylates with in situ silver ion reduction. A polymerizable methacrylate bearing a secondary amino functional group was used to increase the solubility of the silver salt in the hydrophobic resin system. Fourier transform infrared spectroscopy (FTIR) revealed that the silver ion reduction had no significant effect on the degree of vinyl conversion of the methacrylate. X-ray photoelectron spectroscopy (XPS) measurements showed an increased silver concentration at the composite surface compared to the expected concentration based on the total amount of silver salt added. Furthermore, the surface silver concentration leveled off when the silver salt mass fractions were 0.08% or greater. Composites with low concentrations of silver salt (< 0.08% by mass) exhibited comparable mechanical properties to those containing no silver. Transmission electron microscopy (TEM) confirmed that the silver nanoparticles formed within the polymer matrix were nanocrystalline in nature and primarily ≈ 3 nm in diameter, with some large particle aggregates. Composites containing silver nanoparticles were shown to reduce bacterial colonization with as little as 0.03% (by mass) silver salt, while additional amounts of silver salt did not further decrease their surface colonization. With a substantial effect on bacterial growth and minimal effects on mechanical properties, the in situ formation of silver nanoparticles within methacrylate materials is a promising technique for synthesizing antibacterial nanocomposites for biomedical applications.
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http://dx.doi.org/10.1002/jbm.b.31793DOI Listing
April 2011

Investigation of thermally responsive block copolymer thin film morphologies using gradients.

ACS Appl Mater Interfaces 2010 Nov 20;2(11):3241-8. Epub 2010 Oct 20.

Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.

We report the use of a gradient library approach to characterize the structure and behavior of thin films of a thermally responsive block copolymer (BCP), poly(styrene-b-tert-butyl acrylate) (PS-b-PtBA), which exhibits chemical deprotection and morphological changes above a thermal threshold. Continuous gradients in temperature and film thickness, as well as discrete substrate chemistry conditions, were used to examine trends in deprotection, nanoscale morphology, and chemical structure. Thermal gradient annealing permitted the extraction of transformation rate constants (k(t)) for the completion of thermal deprotection and rearrangement of the film morphology from a single BCP library on hydroxyl and alkyl surfaces, respectively. The transformation rate constants ranged from 1.45 × 10(-4) s(-1) to 5.02 × 10(-5) s(-1) for temperatures between 185 and 140 °C for hydroxyl surfaces. For the same temperature range, the alkyl surfaces yielded k(t) values ranging from 4.76 × 10(-5) s(-1) to 5.73 × 10(-6) s(-1), an order of magnitude slower compared to hydroxyl surfaces. Activation energies of the thermal deprotection and film transformation on these surfaces were also extrapolated from linear fits to Arrhenius behavior. Moreover, we noted a morphology shift and orientation transformation from parallel lamellae to perpendicular cylinders at the free surface because of changes in volume fraction and surface energetics of the initially symmetric BCP. Using gradient techniques, we are able to correlate morphological and chemical structure changes in a rapid fashion, determine kinetics of transitions, and demonstrate the effect of surface chemistry on the deprotection reaction in thermally responsive BCP thin films.
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http://dx.doi.org/10.1021/am100695mDOI Listing
November 2010

Structure-activity relationships of antibacterial and biocompatible copolymers.

Biomacromolecules 2010 May;11(5):1286-90

School of Materials Engineering, Purdue University, West Lafayette, Indiana, USA.

The development of polymers that are both bactericidal and biocompatible would have many applications and are currently of research interest. Following the development of strongly bactericidal copolymers of 4-vinylpyridine and poly(ethylene glycol) methyl ether methacrylate, biocompatibility assays have been completed on these materials to measure their potential biocompatibility. In this article, a new methodology for measuring protein interaction was developed for water-soluble polymers by coupling proteins to surfaces and then measuring the adsorption of copolymers onto these surfaces. Ellipsometry was then used to measure the thickness of adsorbed polymers as a measurement of biocompatibility. These results were then compared and correlated with the results of other biocompatibility assays previously conducted on these polymers, affording a greater understanding of the biocompatibility of the copolymers as well as improving the understanding of the effect of hydrophilic and hydrophobic groups that is vital for the development of these materials.
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http://dx.doi.org/10.1021/bm1000839DOI Listing
May 2010

Amphiphile grafted membranes for the separation of oil-in-water dispersions.

J Colloid Interface Sci 2009 Jan 1;329(1):127-32. Epub 2008 Oct 1.

School of Materials Engineering, Purdue University, 701 Stadium Avenue, West Lafayette, IN 47907, USA.

Perfluorinated end-capped polyethylene glycol surfactants were covalently attached to fritted glass membranes as a means to improve the separation of oil-in-water emulsions. Hexadecane was used as representative oil for the oil-in-water emulsions; membrane pore size was varied between 10 and 174 microm. Membranes were characterized with respect to contact angle, permeability of bulk fluids, and separation efficiency. Performance was compared to similar metrics applied to unmodified membranes. Modified membranes demonstrated static hexadecane contact angles which were higher than static water contact angles converse to their unmodified counterparts. The relative hydrophilicity and corresponding oleophobicity of the modified membranes resulted in greater water permeability as compared to hexadecane permeability. The presence of the perfluorinated constituent of the amphiphile retarded the flow of hexadecane. For modified membranes, suspended hexadecane coalesced at the membrane surface, was undercut by water, and floated to the surface such that only trace amounts of oil were present in the permeate. Therefore, modified membranes resisted fouling from oil due to the self-cleaning properties of the attached amphiphile.
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http://dx.doi.org/10.1016/j.jcis.2008.09.068DOI Listing
January 2009

Optimization of silica silanization by 3-aminopropyltriethoxysilane.

Langmuir 2006 Dec;22(26):11142-7

School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA.

Thin films of 3-aminopropyltriethoxysilane (APTES) are commonly used to promote adhesion between silica substrates and organic or metallic materials with applications ranging from advanced composites to biomolecular lab-on-a-chip. Unfortunately, there is confusion as to which reaction conditions will result in consistently aminated surfaces. A wide range of conflicting experimental methods are used with researchers often assuming the creation of smooth self-assembled monolayers. A range of film morphologies based on the film deposition conditions are presented here to establish an optimized method of APTES film formation. The effect of reaction temperature, solution concentration, and reaction time on the structure and morphology was studied for the system of APTES on silica. Three basic morphologies were observed: smooth thin film, smooth thick film, and roughened thick film.
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http://dx.doi.org/10.1021/la061240gDOI Listing
December 2006