PROBING HIDDEN POLYMERIC INTERFACES USINGIR–VISIBLE SUM-FREQUENCY GENERATION SPECTROSCOPY

Understanding the molecular-level processes underlying interfacial phenomena is important in the area of adhesion. We briefly introduce IR–visible sum-frequency generation spectroscopy (SFG) using a total-internal-reflection geometry for the study of polymer–air, polymer–solid, and polymer–polymer interfaces. The following examples, predominantly of work done in our lab, illustrating differences in molecular structure and dynamic properties at interfaces are presented: the air- and solid-interface structure of an amorphous polystyrene (PS) and a semicrystalline polymer with side-chain crystallinity, poly(octadecyl acrylate) (PA-18); structure of a polymer–polymer interface between thin films of a semicrystalline polymer with side-chain crystallinity, poly(vinyl-N-octadecylcarbamate- co-vinyl acetate), and an amorphous PS; thermal order-to-disorder transitions of the air and solid interface of PA-18, and the interface of this polymer with PS; and dynamic surface-relaxation studies of a rubbed PS film.     

Direct Probe of Interfacial Structure during Mechanical Contact between Two Polymer Films Using Infrared Visible Sum Frequency Generation Spectroscopy

Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (PVNODC) and polystyrene (PS) films during mechanical contact. The films were contacted using a deformable semispherical poly(dimethylsiloxane) (PDMS) lens that can be easily adapted to incorporate friction and adhesion force measurements. A strong methyl symmetric peak and Fermi resonance band associated with the alkyl side chains of PVNODC are observed in the SFG spectrum. This suggests that the interface structure during mechanical contact is more ordered than the structure observed for PS/PVNODC bilayer films annealed above the melting and glass transition temperatures of PVNODC and PS. Complementary contact mechanics measurements reveal a small adhesion hysteresis supporting our hypothesis that the interface structure during mechanical contact is not significantly different from the structure of the air interfaces before contact.     

Direct Probe of Interfacial Structure during Mechanical Contact between Two Polymer Films Using Infrared Visible Sum Frequency Generation Spectroscopy

Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (PVNODC) and polystyrene (PS) films during mechanical contact. The films were contacted using a deformable semispherical poly(dimethylsiloxane) (PDMS) lens that can be easily adapted to incorporate friction and adhesion force measurements. A strong methyl symmetric peak and Fermi resonance band associated with the alkyl side chains of PVNODC are observed in the SFG spectrum. This suggests that the interface structure during mechanical contact is more ordered than the structure observed for PS/PVNODC bilayer films annealed above the melting and glass transition temperatures of PVNODC and PS. Complementary contact mechanics measurements reveal a small adhesion hysteresis supporting our hypothesis that the interface structure during mechanical contact is not significantly different from the structure of the air interfaces before contact.     

Welding of polymer interfaces

Studies of strength development at polymer-polymer interfaces are examined and applications to welding of similar and dissimilar polymers are considered. The fracture properties of the weld, namely, fracture stress, σ, fracture energy, G_Ic, fatigue crack propagation rate da/dN, and microscopic aspects of the deformation process are determined using compact tension, wedge cleavage, and double cantilever beam healing experiments. The mechanical properties are related to the structure of the interface via microscopic deformation mechanisms involving disentanglement and bond rupture. The time dependent structure of the welding interface is determined in terms of the molecular dynamics of the polymer chains, the chemical compatibility, and the fractal nature of diffuse interfaces. Several experimental methods are used to probe the weld structure and compare with theoretical scaling laws, Results are given for symmetric amorphous welds, incompatible and compatible asymmetric amorphous welds, incompatible semicrystalline and polymer-metal welds. The relevance of interface healing studies to thermal, friction, solvent and ultrasonic welds is discussed.     

Steady state fluorescence to study nanocomposites based on silica nanoparticles and thermoplastic polymer matrices

Steady state fluorescence was used to study the interfaces of composites formed by silica nanoparticles and three polymers: poly(methylmethacrylate) (PMMA), polystyrene (PS), and low density polyethylene (LDPE). The fluorescent response from the pyrene‐1‐sulfonamide (PSA) was used to study changes appearing in its immediate surroundings. Molecular dynamics of the polymers was studied monitoring the fluorescent response from the PSA as a function of temperature. When the fluorophore was dispersed within the polymers, information from their bulk was obtained while, attaching the fluorophore to the surface of the silica nanoparticles, information from the interface was collected. In the case of the amorphous polymer matrices (PMMA and PS), the presence of silica nanoparticles exerts a small constrain effect by reducing the chain mobility. In the case of the semicrystalline thermoplastic polymer (LDPE) when nanoparticles are not present, only one clear relaxation assigned to the typical diffusion‐like motion of chain segments in the crystallites has been observed. However, when nanoparticles are within the polymer, three relaxations are clearly observed: one in the interlamellar amorphous phase and two due to diffusion‐like motion of chain segments in the crystallites under or without the influence of the nanoparticles, respectively. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Liquid-induced crystallization of poly(ethylene terephthalate)

Amorphous unoriented poly(ethylene terephthalate) was crystallized at 25°C by various organic liquids. The crystalliznity induced in the amorphous polymer was measured by differential scanning calorimetry and infrared spectroscopy. The ability of liquids to interact with and induced crystallinity in the amorphous polymer was classified on the basis of their solubility parameters. Measurements of the density of liquid-crystallized 0.8-mil films of poly(ethylene terephthalate) indicated the presence of extensive internal voids in the semicrystalline polymer matrix. Comparison of differential scanning calorimetric thermograms and infared spectra of heat-crystalized and liquid-crystallized polymer indicated significant differences in the polymer morphologies induced by the two crystallization processes.     

From the glassy state to ordered polymer structures: A microhardness study

The review covers the understanding of the nanostructure development in glassy and semicrystalline polymers as revealed by indentation hardness methods. The microhardness of polymer glasses is discussed emphasizing the influence of thermal history and physical ageing. The correlation between hardness and glass transition temperature is brought in. Furthermore, the role played by the lamellar morphology in the case of amorphous blends of a block copolymer and a glassy homopolymer is highlighted. A discussion on the influence of filler structure on the microhardness of polymer glasses is introduced. Indentation hardness is presented as a valuable tool to study the kinetics of crystallization from the glassy state. As an example, distinct results on polymer systems under different confinement conditions are shown. The nanostructure-microindentation hardness correlation in the case of semicrystalline polymers and the influence of degree of crystallinity and crystal thickness for various flexible and semirigid polymer systems are recalled. A comprehensive discussion of the creep properties of polymer materials is offered. Concerning deformation mechanisms, experimental results show that for polymers with low degree of crystallinity and T_g below room temperature, a large deviation from the microhardness additivity law is always found. This is due to a different deformation mechanism with respect to that envisaged for polymer materials with T_g above room temperature. The assumption that microhardness approaches zero for amorphous materials above T_g is experimentally confirmed. In the case of an oriented material, it is shown that indentation hardness is capable to detect the gradual appearance of phases of intermediate order. In addition, the study of the creep properties also yields valuable information on the internal degree of order of the oriented system. Finally, an overview of the future perspectives of the application of depth-sensing indentation to the study of polymer materials is offered.     

Physical characterization of poly(glycerol sebacate)/Bioglass® composites

Differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and X‐ray diffraction have been used to characterize the structure of a crosslinked polyester (poly(glycerol sebacate), PGS), prepared from a molar ratio of a diacid (sebacic acid) and a triol (glycerol), and their composites formed with an alkaline reactive filler, Bioglass^®. The Bioglass reacts with the sebacic acid carboxylate groups during the composite synthesis, resulting in elastomers that are linked by ionic and covalent crosslinks. Due to its relatively low crosslink density, the unfilled PGS polymer can crystallize below room temperature but is an amorphous elastomer at room temperature. The DSC results show that the Bioglass composites are also semicrystalline below room temperature but the crystallinity is disrupted by the ionic linkages. DMTA of the dry PGS and PGS‐Bioglass composites confirms the semicrystalline nature of the materials and comparison with specimens that had been saturated with water vapour shows that the ionic crosslinks are dissociated by hydration by water molecules. Copyright © 2011 Society of Chemical Industry     

Linear viscoelastic behavior of poly(ethylene terephtalate) above Tg amorphous viscoelastic properties Vs crystallinity: Experimental and micromechanical modeling

Linear viscoelastic behavior of amorphous and semicrystalline poly(ethylene terephtalate), (PET), was experimentally investigated. PET’s samples with different crystallinities (Xc) were prepared and viscoelastically characterized. Based on our experimental results (properties of the amorphous PET and semicrystalline polymers), micromechanical model was used to, first predict the viscoelastic properties of the semicrystalline polymers and second predict the changes on the viscoelastic properties of the amorphous phase when the crystallinity increases. For the micromechanical modeling of semicrystalline material’s viscoelastic properties, difficulties lie on the used numerical methods (Laplace-Carson transformation) and also on the actual physical and mechanical properties of the amorphous phase. In this paper we tried to simplify the Laplace-Carson-based method by using a pseudo-elastic one that avoids the numerical difficulties encountered before. The time-dependant problem is so replaced by a frequency-dependant set of elastic equations. Good agreement with low crystallinity fraction was found however large discrepancies appear for medium and high crystallinity. The poor agreement raises the issue of which amorphous mechanical properties should be taken as input in the micromechanical model? According to the dynamic mechanical analysis (DMA) experimental data, multiple amorphous phases with different glass transition temperatures were observed for each tested semicrystalline sample. For each sample, new glass transition temperature related to an equivalent amorphous phase was determined. DMA tests done at 1 Hz help estimating the mechanical properties of the new amorphous phase based on its new glass transition temperature. Using the new micromechanical approach developed in this paper, the changes occurring on the viscoelastic behavior of the amorphous phase upon crystallization were estimated. Good agreement was found after comparing the micromechanically estimated amorphous behavior with the experimentally estimated one leading to believe that the physical and mechanical properties of the amorphous phase change upon crystallization and taking on account this phenomenon is a key to a good prediction of the semicrystalline behavior using micromechanical models.     

Physical and Mechanical Behavior of Polyamide-6 Containing Oxyaromatic Compounds

Abstract

The distribution of oxyaromatic compound in PA-6 amorphous regions between two structural forms, which are characterized by different energy of interaction with polymer matrix, was studied. Physical and mechanical properties (glass transition temperature, water sorption, elastic modulus) of PA-6 containing oxyaromatic compound were shown to be controlled not only by the net content of oxyaromatic compound in polymer, but by its distribution between these structural forms. This distribution appears to govern the mechanical properties of the polymer in the region of its relaxation transition from glassy to rubbery state, whatever this state was achieved by increasing the concentration of plasticizer (water) or by increasing the temperature. The experimental evidence obtained was discussed with the account for structural heterogeneity of the amorphous regions of semicrystalline PA-6.     

Strain‐induced deformation mechanisms of polylactide plasticized with acrylated poly(ethylene glycol) obtained by reactive extrusion

This work aimed at identifying the tensile deformation mechanisms of an original grade of plasticized polylactide (pPLA) obtained by reactive extrusion. This material had a glass transition temperature of 32.6 °C and consisted of a polylactide (PLA) matrix grafted with poly(acryl‐poly(ethylene glycol)) (poly(Acryl‐PEG)) inclusions. pPLA behaved like a rubber‐toughened amorphous polymer at 20 °C, and its tensile behavior evolved toward a rubbery semicrystalline polymer with increasing temperature. The drawing of pPLA involved orientation of amorphous and crystalline chains, crystallization, and destruction of crystals. It was found that crystal formation and crystal destruction were in competition below 50 °C, resulting in a constant or slightly decreasing crystallinity with strain. Increasing temperature enhanced crystal formation and limited crystal destruction, resulting in an increased crystallinity with the strain level. Drawing yielded a transformation of the initial spherical poly(Acryl‐PEG) inclusions into ellipsoids oriented in the tensile direction. This mechanism may engender the formation of nanovoids within the inclusions due to a decreased density, assumed to be responsible for the whitening of the specimen. © 2015 Society of Chemical Industry     

Relationship between the crystallization behavior and the warpage of film‐insert‐molded parts

The dimensional variation of an injection‐molded, semicrystalline polymer part is larger than the variation of an amorphous polymer part because the shrinkage of a crystalline polymer is generally greater than the shrinkage of an amorphous one. We investigated the warpage of film‐insert‐molded (FIM) specimens to determine the effect of the crystallization behavior on the deformation of FIM parts. More perfect crystalline structures and higher crystallinity developed in the core region of the FIM specimens versus other regions. Relatively imperfect crystalline structures and low crystallinity developed in the adjacent regions of the inserted films, whereas a thin, amorphous skin layer developed in the adjacent regions of the metallic mold wall. The crystallizable substrate in the FIM specimens caused very large warpage because nonuniform shrinkage occurred in the thickness direction of the specimens. Therefore, the warpage of an experimentally prepared FIM poly(butylene terephthalate) specimen was greater than that predicted numerically because of its complex crystallization behavior. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Multiple hydrophilic polymer ultra-thin layers covalently anchored to polyethylene films

A novel procedure has been developed to covalently graft multiple hydrophilic polymer ultra-thin layers to functionalized polyethylene surface. Polyethylene films have been functionalized by two methods, chromic acid oxidation and maleic anhydride grafting, to produce surfaces containing reactive groups, carboxylic acid and anhydride, respectively. The reactive groups formed in the functionalization were used to anchor a poly(vinyl alcohol) (PVA) ultra-thin layer by thermal esterification. After anchoring PVA, a second ultra-thin layer, constituted of poly(acrylic acid) (PAA), was also anchored. The second layer was anchored by thermal esterification between PVA hydroxyl groups and PAA carboxylic acid groups. The procedure presented in this work allows the formation of an ultra-thin layer. The macromolecule anchoring reactions occur only at the interfaces, consequently, only the macromolecules in contact with the interface are anchored. The formation of the ultra-thin layer and the surface characteristics have been analyzed through XPS, ATR-FTIR, SEM, and AFM data.     

Gas antisolvent fractionation of semicrystalline and amorphous poly(lactic acid) using compressed CO2

Semicrystalline and amorphous poly(lactic acid) (l-PLA and d,l-PLA, respectively) were fractionated from chloroform solutions using compressed CO₂ as an antisolvent. The following process variables were used to precipitate normalized molecular weight fractions (NMW) of l-PLA ranging from 0.81 to 1.54 relative to the starting material: polymer concentration, initial organic solution volume, and the rate of antisolvent addition. An analysis of variance (ANOVA) used to quantify the importance of these variables determined that polymer concentration had the most significant impact on the NMW of l-PLA precipitated in this gas antisolvent (GAS) precipitation process. The results of the ANOVA also suggest a predictive approach to polymer fractionation in this complex system. The analysis also highlights the differences and similarities between the fractionation of semicrystalline and amorphous polymers using compressed antisolvents.     

Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process

Use of supercritical carbon dioxide (scCO₂) as a blowing agent to generate microcellular polymer foams (MPFs) has recently received considerable attention due to environmental concerns associated with conventional organic blowing agents. While such foams derived from amorphous thermoplastics have been previously realized, semicrystalline MPFs have not yet been produced in a continuous scCO₂ process. This work describes the foaming of highly crystalline poly(vinylidene fluoride) (PVDF) and its blends with amorphous polymers during extrusion. Foams composed of neat PVDF and immiscible blends of PVDF with polystyrene exhibit poor cell characteristics, whereas miscible blends of PVDF with poly(methyl methacrylate) (PMMA) yield foams possessing vastly improved morphologies. The results reported herein illustrate the effects of blend composition and scCO₂ solubility on PVDF/PMMA melt viscosity, which decreases markedly with increasing PMMA content and scCO₂ concentration. Morphological characterization of microcellular PVDF/PMMA foams reveals that the cell density increases as the PMMA fraction is increased and the foaming temperature is decreased. This study confirms that novel MPFs derived continuously from semicrystalline polymers in the presence of scCO₂ can be achieved through judicious polymer blending.     

The reinforcement of polystyrene and poly(methyl methacrylate) interfaces using alternating copolymers

Asymmetric double cantilever beam studies are presented that document the ability of alternating copolymers to strengthen a polymer/polymer interface. For polystyrene/poly(methyl methacrylate) interfaces, these results show that the alternating copolymer is the least effective sequence distribution of a linear copolymer at strengthening the polystyrene/poly(methyl methacrylate) interface, where the copolymers that are compared all have similar molecular weight and composition. The results also demonstrate that the effect of copolymer molecular weight on the ability of the copolymer to strengthen an interface is controlled by the balance between the increased entanglements and decreased miscibility of the copolymer with the homopolymers with increasing molecular weight.     

Molecular simulations of the solubility of gases in polyethylene below its melting temperature

We have employed Monte Carlo simulations in the osmotic ensemble to study the solubility of three different gases (N₂, CH₄, CO₂) in polyethylene. The simulations are performed at temperatures below the polymer melting point. Although under such conditions, polyethylene is in a semicrystalline state, we have used simulation boxes containing only a purely amorphous material. We show that under such circumstances, computed solubilities are 4-5 times larger than experimental data. We therefore introduce an original use of the osmotic ensemble to implicitly account for the effects of the complex morphology of semicrystalline materials on gas solubility. We have made the assumption that i) the network formed by polymer chains trapped between different crystallites and ii) the changes in local density from crystalline regions to purely amorphous regions, may be both represented by an ad-hoc constraint exerted on the amorphous phase. A single constraint value emerges, independent of the gas nature, characteristic of the crystalline degree of the polymer. It is concluded that the role of this constraint is mostly to reproduce the effective density of the permeable phase of the real material, indirectly giving insights into the morphology of a semicrystalline polymer.     

Effect of the crystallinity and morphology on the microcellular foam structure of semicrystalline polymers

Microcellular foam processing of polymers requires a nucleated cell density greater than 10⁹ cells/cm³ so that the fully grown cells are smaller than 10 μm. A microcellular foam can be developed by first saturating a polymer sample with a volatile blowing agent, followed by rapidly decreasing its solubility in the polymer. In general, the cellular structure of semicrystalline polymer foams is difficult to control, compared to that of amorphous polymer foams. Since the gas does not dissolve in the crystallites (1), the polymer/gas solution formed during the microcellular processing is nonuniform. Moreover, the bubble nucleation is nonhomogeneous because of the heterogeneous nature of the semicrystalline polymer. In this paper, the effects of the crystallinity and morphology of semicrystalline polymers on the microcellular foam processing and on the cellular structure of products are investigated. First, polymer specimens with various crystallinities and morphologies were prepared by varying the cooling rate of the polymer melt. Then, the solubility and diffusivity of the blowing agent in and through specimens were studied. The specimens with differing crystallinities and morphologies were foamed and their cellular structures were compared. The experimental results agree with theoretical predictions, indicating that the crystallinity and morphology of semicrystalline polymers exert a strong influence on the foam processing and the structure of the product.     

Crystallization behavior and mechanical properties of poly(aryl ether ether ketone)/poly(ether imide) blends

Crystallinity and mechanical properties of blends with different amounts of semicrystalline poly(aryl/ ether ether ketone) (PEEK) and amorphous poly(ether imide) (PEI) polymers have been studied. The blends, prepared by melt mixing, have been investigated by differential scanning calorimeter (DSC) to analyze the miscibility between the components and the final crystalline content. Moreover, for the 20/80 PEEK/PEI blend, crystallization in dynamic and isothermal conditions has been carefully investigated in order to find proper conditions for maximum development of crystallinity. Mechanical tests (static and dynamic) have been performed to evaluate the properties of the as-molded and crystallized blends and to compare them with those of crystalline PEEK and amorphous PEI neat resins. Finally, a few SEM observations have been performed to compare the fractured surface of the blend with those of the pure constituents.     

Radical crossover reactions of a dynamic covalent polymer brush for reversible hydrophilicity control

Reversible hydrophilicity control of a radically exchangeable polymer brush with dynamic covalent linkages was successfully demonstrated. A polymer brush with alkoxyamine units was prepared via surface-initiated atom transfer radical polymerization, and reversible surface hydrophilicity control was achieved via dynamic covalent exchange reactions of alkoxyamines. Exchange reactions between alkoxyamine units in the side chains of the polymer brush and the terminal of poly(4-vinylpyridine) (P4VP) were carried out in order to prepare a side-chain functionalized polymer brush. Subsequent quaternization of P4VP chains with iodomethane was carried out to prepare a more hydrophilic surface. In addition, a de-grafting reaction of the quaternized P4VP side chains was performed to confirm reversibility of the alkoxyamine via radical exchange reactions on the surface. All the composition and wettability changes were investigated via X-ray photoelectron spectroscopy and contact angle measurements.