Glass transition and its characteristic length for thin crosslinked polystyrene shells of rodlike capsules

Rodlike capsules consisting of a calcium carbonate core and a crosslinked polystyrene shell were synthesized, and the glass transition temperature (T_g) and characteristic length of the glass transition ξ(T_g) for the thin outer shells were investigated by temperature‐modulated differential scanning calorimetry. The shell thickness ranged from 20 to 129 nm. The ratio of the T_g for the outer shell to the bulk T_g increases with decreasing shell thickness d. The d‐dependence of T_g is interpreted in terms of a simple two‐layer model which assumes that an immobile layer exists near the core‐shell interface. Shells of hollow capsules unexpectedly exhibit a similar d‐dependence of T_g to that for the filled capsules. This is characteristic of the crosslinked polymeric shells, and is attributed to certain spatial heterogeneity of crosslink distribution, and/or to the unstable configuration in the ultrathin shell that does not undergo relaxation due to the crosslink. The latter idea is based on the assumption that unstable configurational state is responsible for the T_g shift from the bulk value observed for nanosized polymeric materials. The ratio of the characteristic length for the shell of the filled capsule to that of the bulk ξ_f(T_g)/ξ_b(T_g) decreases with decreasing d. The results are interpreted in terms of the configurational entropy, and it is also suggested that the configurational state of network polymer chains in the shell affects the characteristic length. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2116–2125, 2008     

Compatibility and crystallization in poly(ethylene oxide)-poly(methyl methacrylate) semi-interpenetrating polymer network

Differential scanning calorimetry together with dynamic mechanical analysis were employed to investigate the crystallinity and the miscibility in poly(ethylene oxide)/crosslinked poly(methyl methacrylate) semi-IPN (interpenetrating polymer networks). The crystallinity of poly(ethylene oxide) in the semi-IPN is found to depend on the crosslink density of PMMA as well as the overall content of PEO. Of special interest is that an increase in the crosslink density tends to increase the crystallinity contrary to our expectation, indicating crystallization and phase separation may proceed simultaneously during IPN formation. The investigation of glass transition behaviors with dynamic mechanical analysis suggests phase separation (i.e., there exist two amorphous phases: one PEO-rich phase, the other a PMMA-rich phase). © 1993 John Wiley & Sons, Inc.     

Strain-hardening modulus of cross-linked glassy poly(methyl methacrylate)

The strain hardening modulus, defined as the slope of the increasing stress with strain during large strain uniaxial plastic deformation, was extracted from a recently proposed constitutive model for the finite nonlinear viscoelastic deformation of polymer glasses, and compared to previously published experimental compressive true stress versus true strain data of glassy crosslinked poly(methyl methacrylate) (PMMA). The model, which treats strain hardening predominantly as a viscous process, with only a minor elastic contribution, agrees well with the experimentally observed dependence of the strain hardening modulus on strain rate and crosslink density in PMMA, and, in addition, predicts the well-known decrease of the strain hardening modulus in polymer glasses with temperature. General scaling aspects of continuum modeling of strain hardening behavior in polymer materials are also presented. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1464–1472, 2010     

Improvement of the thermal stability of cluster‐crosslinked polystyrene and poly(methyl methacrylate) by optimization of the polymerization conditions

The polymerization conditions for polystyrene and poly(methyl methacrylate) crosslinked by 0.5 mol % of the cluster Zr₆O₄(OH)₄(methacrylate)₁₂ were optimized by applying a step polymerization procedure. The onset of thermal decomposition was thus increased up to about 50° for polystyrene and about 110° for poly(methyl methacrylate). The increase in thermal stability correlated with a higher char yield. The glass transition temperatures were also increased by about 15°. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6586–6591, 2005     

Crosslinking PMMA: Molecular dynamics investigation of the shear response

Crosslinking can fundamentally change the mechanical properties of a linear glassy polymer. It has been experimentally observed that when lightly crosslinked, poly(methyl‐methacrylate) (PMMA) has a characteristically more ductile response to mechanical loading than does linear PMMA despite having a higher glass transition temperature. Here, molecular dynamics (MD) simulations are used to investigate conformational and energetic differences between linear PMMA and lightly crosslinked PMMA under shear deformation. As consistent with experiments, crosslinked PMMA is found to have a reduced yield stress relative to linear PMMA. Using the probing capabilities of our explicit atom MD approach, it is observed that while the crosslinks have a minimal direct energy contribution to the total system, they can alter how the main chains conform to macroscopic loading. In crosslinked PMMA, the backbone aligns more with the direction of external loading, thereby reducing the force applied to (and associated deformation of) the polymer bonds. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 444–449     

Incorporation of silver/carbon nanoparticles into poly(methyl methacrylate) via in situ miniemulsion polymerization and its influence on the glass‐transition temperature

Silver/carbon nanoparticles (9 nm) were incorporated, as reinforcements, into a matrix of poly(methyl methacrylate) via in situ miniemulsion polymerization. It was found by differential scanning calorimetry that the glass‐transition temperature of the poly(methyl methacrylate) showed an improvement of 14 °C with only 0.5 wt % nanoparticles in comparison with a pure poly(methyl methacrylate) control, which was also obtained by miniemulsion polymerization under the same conditions. This increase was related to a polymer chain mobility restriction due to a combination of bound plastic and joint plastic shell effects at the interphase and the surrounding regions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 511–518, 2007.     

Viscoelastic characteristics of UV polymerized poly(ethylene glycol) diacrylate networks with varying extents of crosslinking

The viscoelastic relaxation characteristics of ultraviolet crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] have been investigated by dynamic mechanical methods. Effective crosslink density in the networks was varied via the use of PEGDA prepolymers of different molecular weight, or by the introduction of controlled amounts of water in the reaction mixture. In all cases examined, fully amorphous networks were obtained. Time–temperature superposition was applied to obtain master curves of storage modulus versus frequency across the glass transition, and these could be satisfactorily described using the Kohlrausch–Williams–Watts relaxation function. The glass transition temperature (T_g), relaxation breadth, and fragility of the segmental relaxation were correlated with the effective crosslink density obtained in the networks. Gas permeation measurements on the PEGDA/water networks indicated only a very modest variation in gas transport properties, despite the sizeable variation in apparent crosslink density achieved in these materials. This result suggests that the controlling structural factor for gas transport in the networks is not simply crosslink density, and that attempts to correlate gas transport to network structure must necessarily consider the broader relationships between crosslink density, segmental mobility, and fractional free volume. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2058–2070, 2006     

Solution properties,unperturbed dimensions,and chain flexibility of poly(1‐adamantyl acrylate)

Poly(1‐adamantyl acrylate) (PAdA) exhibits much higher glass transition and degradation temperatures than other polyacrylates. However, the quantitative evaluation of the stiffness of this polymer chain has not been reported previously. In this study, the dilute solution properties and conformational characteristics of PAdA were evaluated using viscometry and scattering techniques. The unperturbed dimensions of this polymer were evaluated using the Burchard–Stockmayer–Fixman extrapolation and the touched‐bead wormlike chain model. The PAdA chain has a comparable persistence length, diameter per bead and characteristic ratio to poly(methyl methacrylate) and polystyrene. All these results indicate that PAdA is less flexible than common polyacrylates. In addition, the second virial coefficients (A₂) of PAdA in different solvents obtained by static light scattering were compared. Among the solvents investigated, tetrahydrofuran is a moderate solvent. Radius of gyration of a polymer sample in the various solvents ranged from 16.8 to 30.3 nm. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1526–1531     

The transparency of polyurethane-poly(methyl methacrylate) interpenetrating and semi-interpenetrating polymer networks

Various combinations of polyurethane (PUR) and poly(methyl methacrylate) (PMMA) were prepared as interpenetrating polymer networks (IPN) or semi-IPNs. In the latter, either the PUR or the PMMA component was crosslinked. The optical transmissions of these materials were measured as a function of the crosslink degrees of both phases. The role of the PUR chain extender, poly(oxypropylene) glycol, is discussed. It is concluded that any means which increases the degree of phase dispersion favours the transparency of PUR/PMMA based IPNs and semi-IPNs.     

Crosslinkable polyurethane bearing a methacrylate structure in the side chain

A polyurethane bearing methacrylate groups through urethane linkages was prepared by the addition of 2‐methacryloyloxyethyl isocyanate to the hydroxyl groups in poly(hydroxyurethane) prepared by the polyaddition of a bifunctional cyclic carbonate with 1,12‐diaminododecane. The urethanization proceeded quantitatively in the presence of a catalytic amount of di‐n‐butyltin dilaurate at an ambient temperature, whereas a crosslinked product was obtained from the reaction at 60 °C. The resulting linear polyurethane, bearing a methacrylate structure, was thermally crosslinkable. Its radical copolymerization with vinyl‐type monomers afforded the corresponding crosslinked polymers, whose low glass transition temperatures suggested the flexibility of the polymer chains in the crosslinked product. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3400–3407, 2007     

Phosphorescence of thermally oxidised poly(butadiene) and model enones: temperature dependence

The temperature dependence of phosphorescence of model enone compounds in poly (methyl methacrylate) matrix and glassy methylcyclohexane/isopentane solution and of enones formed from thermal oxidation of poly (butadiene) has been studied over the temperature range 77–220°K. The single discontinuity in the plot for the model enone-glassy solvent is due to the freezing of a (solvent perturbed) barrier to rotation in the enone, and an analogous transition is also observed in the model enone poly (methyl methacrylate) samples and in thermally oxidised poly (butadiene). In the polymer matrices, a second transition corresponding to the γ-transition in poly (methyl methacrylate) and the glass transition temperature T_g in poly (butadiene) are also revealed in the Arrhenius plots. The results demonstrate that care must be taken in ascribing such discontinuities solely to polymer properties since intrinsic properties of the luminescence probe itself may exhibit similar features.     

Volume shrinkage and abrasive wear properties of crosslinked Poly(ester-urethane-acrylate)/MMA copolymer: Effect of chain length

The current study deals with the preparation of novel crosslink transparent poly(ester-urethane-acrylate)/methyl methacrylate (TPEUMA) copolymers and the effect of hydroxyl terminated aromatic polyester (PEs) was also investigated. The properties of TPEUMA were investigated in terms of double bond concentration, polymerization shrinkage, abrasive wear analysis and chemical resistance. The volume shrinkage decreased due to the significant reduction in the concentration of double bonds. The first two effects reflect in lowering the abrasive wear resistance properties at room temperature, while the larger chain length between crosslink decreases the hardness. Decrease in polymerization shrinkage shows more condensed microstructure which was revealed by the abrasive wear rate of TPEUMAs due to hydrogen bonding near to crosslink point. Increase in the chain length of PEs, decreases the glass transition temperature of TPEUMA copolymer which results in loose microstructure. Worn surface were studied using scanning electron microscope to give insight on the wear mechanism of TPEUMA crosslink. It can be suggested from the present study that this copolymer can be used for a broad range of optical applications.     

Development of hydrogenated ring‐opening metathesis polymers

New hydrogenated ring‐opening metathesis polymers with excellent thermal and optical properties were developed. These polymers were prepared by the ring‐opening metathesis polymerization of ester‐substituted tetracyclododecene monomers followed by the hydrogenation of the main‐chain double bond. The degree of hydrogenation was an important factor for the thermal stability of the polymers, and as complete hydrogenation as possible was necessary to obtain a thermally stable polymer. The completely hydrogenated ring‐opening polymer derived from 8‐methyl‐8‐methoxycarbonyl‐substituted monomer has a glass‐transition temperature of 171 °C and a 5% weight‐loss temperature of 446 °C. This polymer has excellent thermal and optical properties because of its bulky and unsymmetrical polycyclic structure in the main chain and is an alternative to glass or other transparent polymers such as poly(methyl methacrylate) and polycarbonate resin. This polymer has also been used in a wide variety of applications, such as optical lenses, optical disks, optical films, and optical fiber. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4661–4668, 2000     

Modulated Differential Scanning Calorimetry Analysis of Interphases in Multi-Component Polymer Materials

A modulated-temperature differential scanning calorimetry (M-TDSC) method for the analysis of interphases in multi-component polymer materials has been developed further. As examples, interphases in a polybutadiene-natural rubber (50:50 by mass) blend, a poly(methyl methacrylate)-poly(vinyl acetate) (50:50 by mass) structured latex film, a polyepichlorohydrinpoly(vinyl acetate) bilayer film, and polystyrene-polyurethane (40:60 by mass) and poly(ethyl methacrylate)-polyurethane (60:40 by mass) interpenetrating polymer networks were investigated. The mass fraction of interphase and its composition can be calculated quantitatively. These interphases do not exhibit clear separate glass transition temperatures, but occur continually between the glass transition temperatures of the constituent polymers. This revised version was published online in August 2006 with corrections to the Cover Date.     

Densely crosslinked polycarbosiloxanes. II: Thermal and mechanical properties

The thermal and mechanical properties of two densely crosslinked polycarbosiloxane systems were investigated in relation to the molecular structure. The networks were prepared from functional branched prepolymers and crosslinked via a hydrosilylation curing reaction. The prepolymers having only vinyl functionalities (poly[phenylmethylvinyl]siloxanes) were crosslinked by using crosslinking agents with reactive silicon–hydrogen groups. In prepolymers having both silicon–vinyl and silicon–hydrogen groups (poly[phenylmethylvinylhydro)]siloxanes crosslinking took place intermolecularly. The thermal and mechanical properties of the polymer networks were found to be dependent on the phenyl  Si O_3/2 (branches) content in the prepolymer, the number of elastically effective crosslinks, the elastically effective network chain density and molecular weight between crosslinks, length of the chain segments introduced by the hydrosilylation crosslinking reaction, and the number of dangling ends. As a consequence of the dense crosslinking, the mechanical properties were also strongly dependent on the glass transition temperature. A tough–brittle transition was observed around the glass transition temperature of the polymer networks. The properties of the poly(phenylmethylvinylhydro)siloxane networks were found to be superior to those of the poly(phenylmethylvinyl)siloxane networks. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1311–1331, 1997     

Synthesis of novel core‐crosslinked graft copolymers from crosslinked poly(mercapto‐thiourethane)

Crosslinked poly(mercapto‐thiourethane) was employed as a precursor for graft copolymer synthesis. The crosslinked stem polymer ( 1 ) was easily prepared by polyaddition of a bifunctional dithiocarbonate and piperazine under air atmosphere via oxidative coupling of mercapto group. Polymerization of styrene and methyl methacrylate in the presence of 1 yielded the corresponding crosslinked graft copolymers with high grafting weight percentages (>1800%). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5097–5102, 2005     

High-resolution NMR of crosslinked polymers: Effects of crosslinked density and solvent interaction

Measurements have been made of the dependence of nuclear magnetic resonance bandwidths of polymers on the degree of crosslinking. Poly(methyl methacrylates) and poly(hexadecyl acrylates) were studied. Three regions of behavior are apparent: (1) in lightly crosslinked materials, bandwidths are quite insensitive to the degree of crosslinking, and the networks behave almost as linear polymers in solution; (2) in moderately crosslinked material, bandwidths are significantly affected by the degree of crosslinking; and (3) in highly crosslinked materials, bandwidths are extremely sensitive to crosslink density, and the polymer peaks become so broad that they disappear almost completely. These results indicate that segmental motion of a polymer in solution is not a function solely of its molecular weight, and that a certain degree of crosslinking is required to restrict polymer motion at the segmental level. The solvent (benzene) peak is always a singlet in swollen poly(methyl methacrylate) systems with swelling ratios up to 6.4 (regions 1 and 2, above) but as the swelling ratio further decreases to 3.5 (region 3), the solvent peak splits into a doublet; this phenomenon may indicate the existence of two different arrangements of solvent molecules in the swollen network, for which interchange is not sufficiently rapid to produce a single line.     

Star polystyrenes by anionic star–star coupling reactions with divinylbenzene

The incremental addition of divinylbenzene was used to generate star polystyrenes in a nearly full conversion of a living polystyrenyl anion. The dramatic increase in the molecular weight of the star polystyrenes with a limited supply of the living polystyrene supported the formation of gradient‐star polystyrenes through star–star coupling. The stoichiometric analysis of the star polymers revealed that their connection polymer had a shorter length than their branch polymer. The measured solution viscosity of the gradient‐star polymers greatly deviated from a linear correlation with the molecular weight and was in parallel to a theoretical simulation based on a highly branched structure of the gradient‐star polymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2579–2586, 2006     

Synthesis and characterization of poly(vinyl alcohol)‐graft‐[poly(D,L‐lactide)/poly(D,L‐lactide‐co‐glycolide)] hydrogels

Poly(D ,L ‐lactide) and poly(D ,L ‐lactide‐co‐glycolide) with various composition and with one methacrylate and one carboxylate end group were synthesized and grafted onto poly(vinyl alcohol) (PVA) via the carboxylate group. The graft copolymers were crosslinked via the methacrylate groups using a free radical initiator. The polymer networks were characterized by means of NMR and studied qualitatively by means of IR spectroscopy. The influence of the glycolide content in the polyester grafts and of the number of ester units in the grafts on thermal properties and swellability were studied as well. The high swellability in water is characteristic of all hydrogels. Differential scanning calorimetry (DSC) showed a single glass transition temperature that occurs in the range between 51 and 69 °C. Thermogravimetric analysis (TGA) of the networks showed the main loss in weight in the temperature range between 290 and 370 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4536–4544, 2007     

Novel doubly polymerizable functional norbornene: Its synthesis,reactivity, and macromolecular architectures from a dual cure via ring‐opening metathesis polymerization and radical photopolymerization

A novel doubly polymerizable functional norbornene, 5‐(methacryloyloxyethylamino carboxylmethyl)bicyclo[2.2.1]hept‐2‐ene (NBMOACM), was prepared. The ring‐opening metathesis polymerization (ROMP) of NBMOACM was carried out to prepare polymers with crosslinkable side chains with the Grubbs catalyst. No gel formation occurred during the ROMP of NBMOACM. The ¹H NMR spectrum of poly(NBMOACM) showed broad signals between 5.10 and 5.40 ppm, corresponding to the vinyl protons of the cis and trans double bonds of the ring‐opened polymer. Increasing the ratio of the monomer concentration to the catalyst concentration resulted in the formation of higher molecular weight polymers. Poly(NBMOACM) was incorporated into poly(methyl methacrylate) [poly(MMA)] to produce AB crosslinked materials. These crosslinked materials [1 wt % poly(NBMOACM), 10% weight loss temperature = 300 °C in air] had higher thermal stability than pure poly(MMA) (10% weight loss temperature = 276 °C in air). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6287–6298, 2006