Peter J. Mills

Analysis of diffusion in polymers by forward recoil spectrometry

We demonstrate that an ion beam analysis method that detects the energies of forward recoiling deuterons can be used to measure concentration profiles and tracer diffusion coefficients D* of a deuterated polymer (d‐polystyrene) diffusing into its hydrogenated analog. The D*’s decrease as M−2 as predicted by the reptation theory of polymer diffusion and agree in magnitude with both the theory and marker displacement measurements of D* in the same system.

Concentration profiles of non-Fickian diffusants in glassy polymers by Rutherford backscattering spectrometry

Rutherford backscattering spectrometry (RBS) is used to determine the concentration profile of 1,1,1 -trichloroethane (TCE) as it diffuses into a crosslinked polymethylmethacrylate (PMMA) glass. The penetration characteristics are those of Case II diffusion, i.e. a diffusion front moves into the glass at a velocity that is roughly constant. Due to the excellent depth resolution (<30 nm) and sensitivity (<500 p.p.m. Cl) of RBS the details of the front can be observed. Behind the front where the concentrationϕ of TCE is high and the PMMA is plasticized to a rubbery state, the concentration gradient of TCE is negligible, indicating that the diffusion coefficient of TCE in this region is greater than 10−9 cm2 sec−1. At the front the concentration of TCE decreases abruptly in less than 100 nm to a lower concentrationϕ0, and subsequently decays exponentially with the distance x from the front. These results are consistent with the Fickian solution to diffusion ahead of a moving boundary, i.e.ϕ(x)=ϕ0exp [− (υx/D)], whereυ is the velocity of the front and D is the (Fickian) diffusivity of the TCE in the glass ahead of the front. These observations are in qualitative agreement with the predictions of a model of Case II diffusion by Thomas and Windle, and a simplified version of their model is proposed.

Differential scanning calorimetry and optical microscopy investigations of the isothermal crystallization of a poly(ethylene oxide)-poly(methyl methacrylate) block copolymer

The isothermal crystallization of the poly(ethylene oxide) block in a linear diblock copolymer of poly(methyl methacrylate) poly(ethylene oxide) with a poly(ethylene oxide) weight fraction of 0.76, has been evaluated using optical microscopy and differential scanning calorimetry. The copolymer was quenched from the melt to a range of crystallization temperatures between 289 K and 316 K and the crystallization monitored by observation of the increase in radius of spherulites (microscopy) or the enthalpy of fusion (calorimetry) as a function of time. Comparison experiments were also made on physical blends of the two homopolymers where the weight fraction of polyethylene oxide ranged from ∼0.6 to 0.9. The block copolymer has an observed melting point which is 2–3 K lower and the spherulite growth rate was reduced compared with the equivalent blend. The growth rates calculated from optical microscopy have been subjected to crystallization regime analysis. All three regimes are observable in the block copolymer for the supercooling conditions used here, only regimes I and II are evident for the pure poly(ethylene oxide), and for the blends regime I appears to be completely suppressed. From the regime analysis a fold surface free energy in the block copolymer of 16–20 erg cm−2 has been obtained, which is much less than that obtained for the pure poly(ethylene oxide) or the blends. An explanation based on the favourable enthalpy of mixing with poly(methyl methacrylate) is suggested. Enthalpy of fusion data from isothermal crystallization studies on all polymers in the d.s.c. have been analysed using Avrami theory. The Avrami exponent was obtained together with an effective rate constant of crystallization. The exponent suggests that crystallization takes place via homogeneous nucleation with a spherical growth morphology, growth being controlled by the rate of attachment of molecules to the interface. By comparison of the Avrami exponent with values obtained for blends differing only in the molecular weight, the influence of melt viscosity on growth control is evident.

Diffusion studies in polymer melts by ion beam depth profiling of hydrogen

Abstract Forward recoil spectrometry (FRES) is used to measure the volume fraction versus depth profiles of undeuterated polystyrene (PS) chains of molecular weight M that have diffused into a high molecular weight deuterated PS matrix. The tracer diffusion coefficient D∗ PS is extracted from these profiles. The D∗ PS is found to be identical with D∗ d-PS , the tracer diffusion coefficient of deuterated PS (d-PS) of the same molecular weight, as well as with D∗ PS measured by other techniques

Real time small-angle X-ray scattering from polystyrene crazes during fatigue

Real time small-angle X-ray scattering (SAXS) from polystyrene (PS) crazed in cyclic three-point bending is investigated using intense X-ray radiation from the Cornell High Energy Synchrotron Source, CHESS. The SAXS patterns are recorded using a two-dimensional image intensifier/TV camera/video tape recorder system, operating at 30 frames/sec. At the maximum of the load cycle the SAXS pattern has a well defined streak normal to the craze fibrils. During the unloading portion of the cycle, however, the streak decreases in intensity and is spread into a diffuse fan. The loss of intensity is due to the decrease in volume of craze matter in the beam as the craze closes while the spreading of the diffraction pattern is due to the disorientation of the craze fibrils as they buckle in response to compression by the surrounding polymer matrix. Whilst reloading of the sample causes a relatively narrow SAXS streak to reappear, at the maximum load irreversible changes occur in the pattern from one cycle to the next. These changes are due to both an increase in craze fibril volume in the beam (craze growth) and to fibril breakdown and permanent disorientation.

The effect of molecular size on non-Fickian sorption in glassy polymers

Rutherford backscattering spectrometry has been used to determine the concentration against depth profiles of n-iodoalkanes diffusing into a polymer glass photoresist. All the iodoalkanes smaller than iodohexane show strongly non-Fickian, or Case II, diffusion. After an induction time a sharp front forms, with almost no concentration gradient behind the front. Ahead of the front the concentration decreases exponentially with depth, a form predicted for Fickian diffusion ahead of a moving boundary. Values of the diffusion coefficientD extracted from this Fickian precursor decrease strongly withn, the number of carbon atoms in the iodoalkane. A similar decrease is observed for the front velocity, the magnitude of which is in qualitative agreement with that predicted by the Thomas and Windle model of Case II diffusion. For the larger values ofn, D decreases asn−2, prompting speculation that these longer chains diffuse into the glass by a reptation-like mechanism.

Near surface damage induced in polyimides by ion beam etching

Polyimide layers approximately 10 μm thick on Si wafers were ion beam etched (IBE) by Ar. Ion energies up to 1000 eV and beam current densities as high as 0.3 mA/cm2 were used to a total dose of 54 mC/cm2. After etching, the samples were exposed to iodine vapor for fixed periods of time. The diffusion of iodine into the samples was used to probe for ion induced changes in the polyimide structure. The concentration of the diffused iodine was measured as a function of depth by Rutherford backscattering spectrometry. For the IBE samples the surface concentration of iodine was markedly decreased. The iodine diffusivity in the near surface region of thickness 0.2 μm was reduced by two orders of magnitude. These results indicate that etching appears to cause modification of the polyimide film at depths far greater than the range of the incident ions or their secondary electrons in polyimide.

Debonding of photoresist caused by Case II diffusion

A glassy polymeric photoresist bonded to a thin copper substrate was immersed in an organic penetrant environment. Debonding of the polymer layer from the substrate was observed by monitoring the deflection of the composite strip. The diffusion of the environment into the polymer layer was followed using Rutherford backscattering spectrometry. For all environments investigated the diffusion showed the characteristics of Case II diffusion, i.e. a uniformly swollen layer formed behind a sharp front and propagated into the polymer at a constant velocity. Even though the front velocity could be varied over three orders of magnitude by varying the environment or the temperature, debonding always occurred when the front had penetrated only about one-fifth of the total layer thickness. It is concluded that debonding is driven by release of the elastic strain energy stored in the composite strip rather than a specific attack of the interface by the environment. Additions of a smaller organic molecule to a predominately large organic molecule environment were found to produce a marked increase in the kinetics of debonding and a corresponding increase in the Case II front velocity. To discover the mechanism of this effect, experiments were carried out with mixtures of iodomethane and 1,1,1-trichloroethane (TCE). Rutherford backscattering spectra showed that the smaller iodomethane diffused ahead of the main Case II diffusion front of the TCE. It is proposed that the increase in Case II front velocity in the mixture results from the fact that the faster diffusing iodomethane preplasticizes the polymer ahead of the front.

Ion Beam Analysis of Diffusion in Polymer Melts

Two ion beam depth profiling methods have been used to measure the diffusion of polymer chains of molecular weight M into a matrix of polymer of molecular weight P. In the first the displacement xm of Au markers at the original interface of a diffusion couple between polystyrene with P=2×10 7 and a thin film of PS with M 0 5 , where D* the tracer diffusion coefficient of the M chains at 174°C, is found to be D*=O.007M −2 cm 2 /sec, in good agreement with the D*=DR expected for the reptation mechanism. Forward recoil spectrometry, a technique in which the energies of recoiling deuterons are detected, is used to obtain concentration profiles, and hence D*, of deuterated PS M-chains diffusing into a hydrogenated PS P-chain matrix. When P>>M, D*=0.008M −2 , in good agreement with the marker data. When P e 2 /(Mp 3 ) describes the diffusion of the M-chain by release of its topological constraints (by diffusion of the surrounding P-chains) and M e is an entanglement molecular weight. D* for self-diffusion (M=P) is dominated by reptation except for Ms close to M e .