Stephen J. Spells

Confocal Raman Depth Profiling of Surface-Modified Polymer Films: Effects of Sample Refractive Index

A solution-cast poly(vinyl chloride) (PVC) film of thickness 81 μm was modified with aminothiophenol using surface-selective reaction conditions. After the identification of suitable Raman bands characteristic of the modifier (1596 cm−1) and the polymer (1425 cm−1), confocal Raman measurements demonstrated a depth variation of the modifier peak intensity which was consistent with a highly surface-selective reaction. Data also showed features clearly attributable to refractive index effects, principally the lack of symmetry in the modifier depth profile obtained from the raw data and an underestimated film thickness. The film was cross sectioned under liquid nitrogen to allow conventional lateral Raman measurements across its thickness. The depth profile obtained directly in this way was in good agreement with confocal data after normalization using the polymer peak intensity and an expansion of the thickness scale to the value obtained independently with a micrometer. The nondestructive confocal method has clear advantages in analyzing small samples or where spatial mapping is also required. To simulate the effect of sample refractive index, the directly measured cross-sectional depth profile was used to calculate the confocal depth profile, using the method of analysis developed by Everall.1,2 The general features of the profile were reproduced, assuming a “fill factor” for the 100× microscope objective of 0.63. However, a broadening of features was apparent in the simulated profile. This suggests that direct conventional Raman measurements of a sectioned film may provide a more sensible approach for the detection of interfaces in polymer films.

Surface Modification of Polystyrene Films. Depth Profiling and Mapping by Raman Microscopy

The chemical modification of polymer films was studied using Raman microscopy. The nitration of polystyrene films (PS) in a non-swelling reaction medium was compared with the nucleophilic substitution of chlorine atoms by cyano groups in poly(p-chloromethylstyrene) (PSC) films in mixtures of a good and a non-solvent for the polymer. Using Raman microscopy in the confocal mode, depth profiles of films modified in different conditions were recorded. In both reactions, conditions could be found which allowed the polymer films to be preferentially modified at the surface. The gradient of the degree of modification and the surface selectivity are functions of reaction time, temperature and solvent quality. The homogeneity of the modification reactions was compared by mapping of surface areas using Raman spectroscopy in the conventional mode.

Non-integer and mixed integer forms in long n-alkanes observed by real-time LAM spectroscopy and SAXS

Application of real-time Raman longitudinal acoustic mode (LAM) spectroscopy is demonstrated in the studies of comparatively fast crystallising and transforming monodisperse polyethylene oligomers. The experiments have confirmed the structural model of the transient non-integer folded (NIF) form in long-chain n-alkane C246H494 derived previously from real-time small-angle X-ray scattering (SAXS). During crystallisation of the NIF and its subsequent isothermal transformation into the once-folded form F2 a single fundamental (LAM-1) peak of constant frequency is observed, which corresponds to the straight chain segment length exactly equal to half the full chain length. This confirms that crystalline core layers of the NIF have a constant thickness corresponding to that of the once-folded form F2. The remaining thickness making up the full lamellar periodicity thus consists of an amorphous layer made up of cilia. The constant frequency, but increasing intensity of the LAM-1 band during isothermal annealing is consistent with the cilia crystallising by re-entering the existing crystalline layers. In alkanes C210H422 and C198H398 another mechanism of transformation of the NIF into a more stable structure is observed. A new mixed-integer folded-extended (FE) phase forms instead of the pure once-folded F2 phase. Instead of re-entering the existing crystal layer, the cilia from two neighbouring NIF lamellae gather and form a third intracalated crystalline layer. The resulting superstructure contains three crystalline layers in a repeat unit. Whether one or the other of the two, transformation modes take place appears to depend on factors such as fraction of folded chains in the NIF, overcrowding at the crystal-amorphous interface and chain diffusion

Depth profiling of modified PVC surfaces using confocal Raman microspectroscopy

Depth profiles of poly(vinyl chloride) (PVC) films chemically modified in solvent/non-solvent mixtures were obtained using confocal Raman microscopy (CRM). The non-destructive technique allows the characterization of interphases of up to 50 microns in thickness with a resolution of 1.4 mu m. The accuracy of CRM is demonstrated by comparison of mean degrees of modification calculated from depth profiles and values obtained by transmission FTIR spectroscopy.

Structural information from progression bands in the FTIR spectra of long chain n-alkanes

The low temperature FTIR spectrum of long chain n-alkanes has been investigated in the region between the C–C stretching and CH2 twisting fundamentals (1050–1133 cm−1). With successive annealing and cooling stages, extended chain crystals of n-C198H398 show an improvement in the regularity of the progression bands observed. This is related to a ‘perfecting’ of the crystals. A once-folded sample of the same alkane shows additional features between 1050 and 1100 cm−1, attributed to resonance modes from a tight (110) fold. These disappear on transformation to the extended form, to be replaced by progression bands. Assignment of the individual bands enables the length of the all-trans chain to be estimated and this method is used to show that centre-branched long chain n-alkanes have a folded conformation. It is also shown that the chain length derived from such FTIR data for a 1:1 molar mixture of n-C162H326 and n-C246H494 is consistent with a triple layer superlattice structure.

Structural and morphological features of the syndiotactic polystyrene/ethylbenzene complex

The δ-phase of the syndiotactic polystyrene/ethylbenzene system, as formed by crystallisation from dilute solution, is shown to result in a lamellar morphology, with a monoclinic crystal lattice similar to that already reported for the complex with toluene. Combined SAXS, FTi.r., d.s.c. and t.g.a. measurements show that an increase in the X-ray long period between 80 and 120°C is related to solvent loss and decomplexation. There is also some temporary loss in helical order. SANS measurements on the δ-phase reveal negligible isotopic fractionation. The small increase in the in-plane radius of gyration with increasing molecular weight is interpreted as evidence of a sheetlike molecular conformation, with superfolding at higher molecular weights leading to a multiple sheet structure. On transformation to the solvent-free γ-phase, an increase in radius of gyration results from a displacement of crystal stems out of the δ-phase sheets.

Correction of diffraction effects in confocal raman microspectroscopy

A mathematical approach developed to correct depth profiles of wet-chemically modified polymer films obtained by confocal Raman microscopy is presented which takes into account scattered contributions originated from a diffraction-limited laser focal volume. It is demonstrated that the problem can be described using a linear Fredholm integral equation of the first kind which correlates apparent and true Raman intensities with the depth resolution curve of the instrument. The calculations of the corrected depth profiles show that considerable differences between apparent and corrected depth profiles exist at the surface, especially when profiles with strong concentration gradients are dealt with or an instrument with poor depth resolution is used. Degrees of modification at the surface obtained by calculation of the corrected depth profiles are compared with those measured by FTIR-ATR and show an excellent concordance.

The application of mixed crystal i.r. spectroscopy to molecular deformation in polyethylene by rolling

The mixed crystal i.r. technique has been applied to polyethylene single crystal mats after rolling at room temperature to roll ratios up to 6.0. Reduction in the absorbance of the outer CD2 bending components provides evidence of lamellar breakup. Model calculations indicate that the size of blocks which break away from the crystals is reduced to approximately 36 angstrom for a ratio of 6, while X-ray diffraction data show that crystallite reorientation occurs progressively with increasing roll ratio. Changes in small-angle X-ray scattering and the Raman longitudinal acoustic mode with rolling demonstrate that crystallite stacking periodicity is gradually lost and that crystal thickness remains unchanged.

On Perpendicular and Tilted Chains in Lamellar Crystals

Chain tilt and surface disorder were investigated in end-deuterated long n-alkane C12D25C192H384C12HD24 crystallized from solution and in n-alkane C162H326 crystallized from melt. Small-angle X-ray scattering and infrared spectroscopy were employed. Extended-chain crystals of C12D25C192H384C12HD24 as-grown from solution have the molecular axis perpendicular to the lamellar surface, but when heated, around 90°C, they start tilting relative to the layer normal. The tilt increases gradually to reach 35° just below the melting point. C162H326 crystallized from the melt at small supercoolings has chains tilted at 35° at the outset, as found previously for all melt-crystallized long alkanes and polyethylene. However, for the first time in long alkanes, it is found that when molten C162H326 is supercooled to ΔT≥10 K, crystals with perpendicular chains form. At still larger ΔT, the chains are once-folded, with a mixed population of tilted and perpendicular chain crystals. The use of Davydov splitting of the CH2 and CD2 bending vibration of the end-labelled alkane C12D25C192H384C12HD24 allows independent IR probing of molecular disorder at the deuterated surface and in the hydrogenous crystal interior. The initially small CD2 splitting and the presence of an additional singlet component indicate a rough surface in as-grown crystals, with considerable longitudinal interchain disorder. It is estimated that about 10% of chains are displaced by up to a dozen C-atoms. The increase in splitting and decrease in absorbance of the singlet, which occur on annealing at progressively higher temperatures, are evidence of steady improvement in translational surface order, occurring simultaneously with increasing chain tilt angle. From the above evidence, it is concluded that the absence of tilt in as-grown crystals is not the result of high surface order, as in the case of shorter odd n-alkanes, but rather of a high frozen-in longitudinal disorder with chain ends or folds protruding out of or sunken beneath the crystal surface. It is also concluded that chain tilt only becomes necessary as the crystal surface becomes translationally more ordered and the crystal–amorphous interface sharpens. The effect of chain tilt on the Davydov splitting is addressed briefly.

The effect of short chain branching on local chain organisation in isotopically labelled blends of polyethylene

The complementary techniques of small angle neutron scattering (SANS) and infra-red spectroscopy have been used to determine features of molecular trajectory for isotopic blends incorporating linear polyethylene and copolymers containing butyl or hexyl branches. SANS data show both a more compact conformation for a copolymer guest molecule than for a linear guest and also a smaller molecular expansion with increasing molecular weight when both guest and host are copolymers. These blends also show the smallest proportion of {110} isolated guest stems, while wholly linear blends show the largest proportion, on the evidence of the infra-red CD2 bending band profiles. Estimates are made of the sizes of ‘groups’ of adjacent stems, and these also show a corresponding dependence on the sample type, indicating a higher proportion of adjacent re-entry for copolymer blends than for linear blends.