Change of cBT grain size on the extraction process

Figure 23: Relative distribution of edge lengths in processed CBT100 according to optical microscopy: a) CBT100 as delivered, b) thoroughly and c) roughly manually grinded in a mortar, d) crumbled between steel plates and e) treated in a rotor speed mill.

The extractions were carried out on a scale of 130 g CBT100. It was placed in a frit with a bottom of fused silica of porosity G0 with CY as solvent. G0 was chosen despite the better yield of G2 because of its lower amount of opened cycles in the extract and the less extensive clogging of the frit allowing a more appropriate comparison of the results (chapter 5.1.4.4, p. 60 ff.). The used cBT types were: cBT finely or roughly grinded in a mortar, cBT granulate just crumbled between steel plates, a mix of roughly grinded cBT and non-processed cBT granulate in a ratio of 1:1 or of cBT granulate mixed with PBT granulate in a ratio of 2:1 (Table 2). The enhancement of solvent flow by passing an argon (Ar) stream through the cBT in the frit by means of a cannula was explored in an additional experiment but did not have an effect on the yield, as expected.

a

d

b c

e

Table 2: Conditions of the extractions of cBT in various grain sizes (in a frit with a porosity G0 and a length of 12 cm).

Exp.

no.

Mass of used cBT [g]

Processing of cBT

Additional modification

Time of extraction [h]

Yield of extract [%]

1 130 grinded in mortar 40 1.5

2* 330 grinded in mortar 40 0.3

3

65 grinded in mortar

24 21.4

65 CBT100

4 114 grinded in mortar PBT pellets (63 g) 24 7.7

5 131 roughly grinded 24 55.6

6 130 crumbled

20 39.0

44 43.5

7 130 milled Ar flow 9 0.8

*Usage of a longer frit of 26 cm with porosity G2.

5.1.4.5.1 The extracts

LM-cBT was obtained by extraction of all evaluated grain sizes with melting temperatures of about 160 to 166 °C although it was accompanied by an additional high-melting fraction in smaller-scale extraction when using grinded cBT (exp. no. 1 in Table 2). The yield was dependent on the grain size and its distribution as expected (Figure 24a). This also applies for the content of opened cycles and the oligomeric ratio (Figure 25). Two samples of every fraction were measured by DSC (except for the low-yielding extractions with just grinded cBT in different scales, exp. no. 1 and 2 in Table 2) with a close resemblance. The results of only one sample are depicted. A high amount of small cycles was generally obtained in the extracted fractions compared to CBT100 and to the residues after extraction (according to the ratio of the aromatic signals in ¹H NMR and MALDI-TOF). Only in case of the product from extraction of crumbled CBT100 (exp. no. 6), a lower oligomeric ratio that in CBT100 is observed. No correlation was observed between the oligomers in the extracted fractions detected by MALDI and the yield of the fractions nor their melting temperatures. All MALDI spectra of the extracts were dominated by the peaks of the tri- to pentamer. Signals of the tetramer were absent in some extractions (exp. no. 4 and 6). The hexamer was additionally observed in the extraction of coarsely grinded cBT (exp. no. 5), the one with the highest yield.

Extractions with a higher yield exhibit minor signals of impurities in MALDI spectrometry and ¹H NMR spectroscopy. The impurities in CBT100 may have preferably been extracted at the beginning. The low yield extraction of finely grinded CBT100 (without any added “fillers”

like cBT or PBT pellets, i.e. exp. no. 1 and 2) resulted in an extract with decomposition products. Additional peaks were noticed in ¹H NMR as well as a yellowish color and an acrid odor similar to that of thermally degraded PBT.

The amount of opened cycles calculated from NMR spectra are higher than in CBT100 (>8 vs. 2 mol-%). The utilization of equipment with a frit with pore size G2, which was previously found to lead to a superior yield (at the expense of further impurities) (chapter 5.1.4.4), and of a larger scale (exp. no. 2) actually “improved” the melting behavior of the extract significantly but did not result in an improvement of yield (Figure 24a, exp. no. 1 and 2).

Figure 24: DSC-based analysis of the extraction of differently processed CBT100

a b

total volume was higher when additional PBT was present. This additional mass had to be passed and heated by the solvent, which probably caused the different performance. Very similar thermograms were observed for both experiments. ¹H NMR spectra proved the presence of impurities especially in case of added cBT granulate. The ratio of hydroxy termini is significantly lower in both cases than in the extractions of only grinded cBT or in non-treated CBT100 itself (0.4 or <0.1 mol-% for addition of CBT100 or PBT compared to 8.7 mol-% for grinded cBT and 1.9 mol-% for CBT100) (Figure 25a).

Extraction of cBT in coarse grain (only roughly grinded or crumbled CBT100) resulted in a further increase of the yield of LM-cBT. The amount of linear chains is reduced below the detection limit of ¹H NMR spectroscopy (Figure 25a, column 5 to 7). Results of the extraction of crumbled cBT were evaluated after 20 and after 44 h to estimate the effect of prolonged extraction. The melting endotherms were similar but tended to higher values at longer extractions. The space-time-yield decreased drastically during the prolongation (39.0 wt-%

after 20 h, 4.5 wt-% after additional 24 h) as observed before in this study (Figure 24a).

The extraction enhanced by an Ar stream for loosening of the extraction material resulted in the mentioned low yield (Figure 24a) and in a small but noticeable amount of linear chains in the extract (Figure 25a, column 8). The latter is probably caused by the degradation observed for extractions of finely powdered cBT.

Figure 25: Results of ¹H NMR spectroscopy evaluation for the extractions of differently processed CBT100 (numbers indicate exp. no. from Table 2) in respect of a) the fraction of hydroxyl end groups (determined by the ratio of -CH2-OH) and b) the oligomeric ratio (according to aromatic signals).

a b

5.1.4.5.2 The residues after extractions

The melting temperature of the cBT residues after extraction was generally higher if a noticeable yield of extracted fraction was obtained (Figure 26). (The correlation of highest melting temperature and yield are depicted in this figure for extractions with at least 50 g of extraction material (from chapter 5.1.4.3, 5.1.4.4 and this chapter) to avoid scale-related effects in smaller batches.) The residue’s melting temperature in dependence of the yield are probably caused by the complex formation of the melting endotherms by interaction of the cBT oligomers amongst each other.

Figure 26: The melting temperature of the residues after extraction in dependence of the yield of cBT extracted therefrom for extractions of 50 g or more CBT100.

MALDI-TOF spectra of samples from the residues showed signals from larger cycles than observed in the extracts. They consisted mainly of signals for cyclic tri- to heptamers with very small peaks for oligomers with 11 or less repetition units. Small but identifiable signals of linear impurities were observed with 8 or less repetition units, which were already observed in CBT100 (chapter 5.1.1.3, p. 39 ff. and Figure 3). The extraction with the highest yield (coarsely grinded cBT, exp. no. 5) resulted in MALDI spectra with a reduced signal of cyclic trimer compared to the other spectra. This can be a hint for a preferred extraction of

The maze of the outer layer appeared denser in case of high yields than for those with lower yields observed and described. Its crystalline plates had edge sizes in the range of 3 – 17 µm and were partly frayed in all cases regardless of the yield. No relation between the yield and the thickness of the outer layer was found.

Figure 27: SEM images of the residue after extraction a) of finely grinded cBT (exp.

no. 1) and b) of finely grinded cBT in presence of PBT (exp. no. 4).

Figure 28: SEM images (all by InLens detector) after extraction of crumbled cBT (exp.

no. 6) a) of the 4 layers in overview (indicated by Roman numbers), b) of the outer and the additional layer beneath and c) of the intermediate layer (III).

The inner phase is similar to CBT100 in all cases with its small crystalline plates in an amorphous matrix as observed and described above. It is followed by the intermediate layer described before. Its thickness increases with increasing yield of dissolved cBT (Figure 29).

This is the same dependency of the thickness as observed before (chapter 5.1.4.4, p. 60 ff.).

The thickness however varies between samples of one probe and they appear to decrease

30 µm 20 µm

a b

40 µm

6 µm

3 µm

a b

c

I II III IV

with depth in the frit (tested for the residue in the frit after the extraction of crumbled cBT). It hence suggests a decrease of the extent of extraction with depth in the extraction vessel.

The corresponding DSC thermograms were measured of residue samples from different heights in the frits in case of the extraction of finely grinded cBT (exp. no. 1, two samples) and of crumbled cBT (exp. no. 6, three samples) (Figure 24b). However, the melting temperature is the product of a complex interaction of the different oligomers and could obviously not be measured of just one layer. A decrease of the degree of extraction has been expected in the setup used. The (comparably small) solubility of cBT in the solvent is reached while the solvent passes through the frit.

Figure 29: Thickness of the intermediate layer in SEM images of the residues after extraction in dependence of the yield of cBT extracted therefrom for extractions of 50 g CBT100 or more.

The formation of an outer layer consisting of crystalline plates without amorphous matrix can be observed in optical microscopy as well. A crystalline glitter after extraction can be distinguished from the appearance from before (example images in Figure 30 and Figure

yield of >55 wt-% (factor to initially found average length of 0.6 – 0.8)). A gain with an average factor between 1.2 and 3.3 was found in all other extractions. The increase during extraction of crumbled cBT (exp. no. 6), where a comparably high yield of about 46 wt-% was obtained, was moderate compared to the other grain sizes (factor of 1.2 – 1.8).

Figure 30: Optical microscopy images of a) the residue after extraction (exp. no. 5) and b) coarsely grinded cBT from before.

Figure 31: Optical microscopy images of a) the residue after extraction (exp. no. 6) and b) crumbled cBT from before.