Introduction to Polymer Science and Technology

Thermal properties

combine together with the curve representing as-moulded specimen as the test temperature approaches the material melting point. The specimen with the highest crystallinity, as identified with the highest annealing temperature, requires the highest temperature to reduce in stiffness to the same level of E' as that of as-moulded specimen. The associated tan 5 values exhibit significant reductions in the peak heights, and also indicate an increase in T .

Figure 7.18Storage modulus (E') vs. temperature for Nylon 46: as-moulded and heat treated at 135,150 and 180 °C (source: Sepe 1997, p29)

Figure 7.19 shows the influence of intermolecular forces of attraction on the storage modulus: as the H-bonding increases, the modulus increases across the temperature range and the T also increases. The associated tan 5 curves (not shown here, see Ehrenstein 2004, p279), as well as showing the shift in T , also show a decrease in the tan 5-peak height from the highest for PA 6, then PA 66, and to the shortest for PA 46. The tan 5-peak height is an indicator of the amount of amorphous proportion or the degree of crystallinity in semicrystalline thermoplastics. Therefore, PA 6 is the least and PA 46 is the most crystalline of these polyamides. This is also born out by the melting points of these materials, which are about 220, 260 and 290 °C for PA 6, PA 66 and PA 46, respectively.

-100

100 200

Temperature, °C

Figure 7.19Storage modulus (G') vs. temperature for polyamides. The tests are conducted in torsion at 1 Hz frequency and 3 °C/min heating

rate (source: Ehrenstein 2004, p279)

Introduction to Polymer Science and Technology Thermal properties

The intermolecular forces of attraction between the neighbouring polymer chains in polyamides arise from the hydrogen bonding between the carbonyl oxygen and the amide hydrogen atoms of the amide groups (links) within the polymer backbone chain. The strength of the attraction depends on the frequency of the amide groups along the chain, i.e., the ratio of amide group (NHCO) to CH₂, and as it increases so does the level of H-bonding. In the case of PA 6, [- (CH₂)₅ - CONH - ]_n, the amide links, therefore the H-bonds, are separated by five methylene (or methene) groups; in PA 66, [- NH - (CH₂)₆ - NHCO - (CH₂)₄ - CO -]_n, by six and four groups; and in PA 46, [- NH - (CH₂)₄- NHCO - (CH₂)₄ - CO -]_n, by four and four (- CH₂ - ) groups. As can be seen even such, seemingly, small variations in the chemical structure makes a clear difference in the thermal properties discussed.

Figure 7.20 shows storage modulus (E') curves over a range of temperature for polyethylene terephthalate (PET) fibres in as-spun and drawn at draw ratios of 2 to 5.5. Drawing in fibrescauses uniaxial molecular orientation and increases crystallinity. Drawing, as well as causing increase in crystallinity also brings about a degree of alignment and reduction in free volume within the amorphous regions and therefore causes a level of constraint to the mobility of the molecular-chain segments. Such structural variations in turn bring about significant changes in mechanical and physical properties, e.g., stiffness and strength increase, T and T increase, damping reduces, etc.

О > О > g _m > Г О >

Introduction to Polymer Science and Technology

Thermal properties

110 150

temperature, °C

Figure 7.20Storage modulus (E') vs. temperature for PET fibres drawn at various draw ratios. The draw ratios are indicated on the curves

(note that 1 dyne/cm² = 0.1 Pa) (source: Miller 1984)

The increase in stiffness can be clearly seen by the increases in E' with increasing draw ratio (molecular orientation), the glass-transition region broadens and becomes shallower, i.e., the extent of drop in stiffness between the glassy and rubbery states is much reduced. The changes indicated in the transition region are also present in the associated tan 5 curves: as-spun fibre specimen exhibits the sharpest and the highest tan 5 peak that occurs at the lowest temperature, and the tan S curves then, successively, get shallower, broader and move to higher temperatures with increasing molecular orientation.

7.4.1.5 Effect of crosslinking

Crosslinking has a dramatic effect on dynamic mechanical properties, particularly at and above T , as illustrated in Figure 7.21. With increasing crosslinking (indicated by the arrow on the figure), glass transition occurs over a broader temperature range and the associated drop in the storage modulus becomes much reduced, and at glass transition the damping peaks reduce in height (i.e., the damping intensity decreases), broaden and shift to higher temperatures.

These features were observed in the thermosetting acrylic systems that I studied for my PhD, some four decades ago at UMIST, under the supervision of the late Dr Eric White, a very kind and caring man, who set me off on my career. I struggled a while trying to do tests using a laboratory made vibrating reed apparatus and was, then, saved by the arrival of the Rheovibron, one of the first commercial DMTA equipment. The material tested was an acrylic system with hydroxyethl acrylate (HEA) comonomer to facilitate crosslinking with hexamethoxymethlmelamine (HMMM). By adjusting the contents of these two ingredients, it was possible to obtain acrylic thermosets of various degrees of crosslinking. The DMTA tests were conducted on these polymer systems, using specimens in the form of rectangular film strips of approximately 30 x 2 x 0.03 mm, at 110 Hz in tension. Figure 7.22 shows plots of E' and tan 5 against temperature for formulations containing different levels of the cross-linking agent of HMMM as a % of the HEA comonomer.

Date: 2015-12-11; view: 755