Showing papers by "James A. Forrest published in 2001"

Molecular weight dependence of reductions in the glass transition temperature of thin, freely standing polymer films.

Abstract: We have used transmission ellipsometry to perform a comprehensive study of the glass transition temperature ${T}_{g}$ of freely standing polystyrene films. Six molecular weights ${M}_{w},$ ranging from $575\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $9100\ifmmode\times\else\texttimes\fi{}{10}^{3},$ were used in the study. For each ${M}_{w}$ value, large reductions in ${T}_{g}$ (as much as $80\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ below the bulk value) were observed as the film thickness h was decreased. We have studied in detail the dependence of the ${T}_{g}$ reductions on ${M}_{w}$ in a regime dominated by chain confinement effects. The empirical analysis presented is highly suggestive of the existence of a mechanism of mobility in thin freely standing films that is inhibited in the bulk and distinct from the usual cooperative motion associated with the glass transition.

Swelling of Poly(dl-lactide) and Polylactide-co-glycolide in Humid Environments

Abstract: The water uptake of poly(dl-lactide) (PLA) and its copolymer with glycolic acid (PLGA 50:50) has been studied using a quartz crystal microbalance (QCM). The polymers were swollen in humid atmospheres at 20 °C in order to probe both the kinetics of adsorption and the equilibrium amount of water taken up at low concentrations. The relative humidity of the air was controlled using saturated salt solutions. By equating the chemical potentials of the water vapor and the polymer/water mixture at equilibrium, it was possible to extract the Flory−Huggins interaction parameter χ from a simple binary mixing theory. Studies were also performed on the formulations of these materials with a model polypeptide active ingredient. The kinetics of swelling were described using a model based on the Thomas−Windle model of penetrant diffusion in glassy polymers.

Crystal growth rate in ultrathin films of poly(ethylene oxide)

Abstract: Many dynamical properties of polymers, including segmental relaxation and chain diffusion, exhibit anomalies in thin-film samples. We extend the studies of thin-film dynamics to the case of semicrystalline polymers and present a study of the crystal growth rate for thin films of poly(ethylene oxide). We used optical microscopy and quartz crystal microbalance techniques to characterize the kinetics of crystallization for films with thicknesses from 40 to 1000 nm for a range of temperatures near the melting point. A remarkable slowing down of the crystal growth is observed at all temperatures studied for films with a thickness of less than ∼100 nm. The results can be used to suggest reductions of the mobility of chains at the crystal/amorphous interface.

Sub‐glass‐transition temperature interface formation between an immiscible glass rubber pair

Abstract: We used neutron reflectivity to measure the interfacial width in the immiscible system polystyrene/poly(n-butyl methacrylate) (PS/PnBMA). Measurements were made on the same samples at temperatures ranging from below the glass-transition temperature (Tg) of PS to slightly above. We observed significant broadening of the interface at temperatures below the Tg of PS, indicating chain mobility below the bulk Tg value. The interfacial width exhibited a plateau at a value of 20 A in the temperature range of 365 K < T < 377 K. A control experiment involving hydrogenated and deuterated PS films (hPS/dPS) showed no such broadening over the same temperature region. The results are consistent with a reduction of the Tg of PS in the interfacial region of ∼20 K. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2664–2670, 2001

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Abstract: Man ydynamica lpropertie so fpolymers ,includin gsegmenta lrelaxation an dchai ndiffusion ,exhibi tanomalie si nthin-fil msamples .W eexten dth estudie sof thin-fil mdynamic st oth ecas eo fsemicrystallin epolymer san dpresen ta stud yo fthe crysta lgrowt hrat efo rthi nfilm so fpoly(ethylen eoxide) .W euse doptica lmicroscopy an dquart zcrysta lmicrobalanc etechnique st ocharacteriz eth ekinetic so fcrystalliza- tio nfo rfilm swit hthicknesse sfro m4 0t o100 0n mfo ra rang eo ftemperature snea rthe meltin gpoint .A remarkabl eslowin gdow no fth ecrysta lgrowt hi sobserve da tall temperature sstudie dfo rfilm swit ha thicknes so fles stha n;10 0nm .Th eresult scan b euse dt osugges treduction so fth emobilit yo fchain sa tth ecrystal/amorphous interface . ©200 1Joh nWile y& Sons ,Inc .J Poly mSc iPar tB :Poly mPhy s39 :2615-2621 ,2001