Latest Developments in GPC Analysis of Adhesive and Sealant Polymers Mark Pothecary PhD Americas Product Manager Malvern Instruments

Transcription

1 Latest Developments in GPC Analysis of Adhesive and Sealant Polymers Mark Pothecary PhD Americas Product Manager Malvern Instruments

2 Molecular weight The most fundamental molecular property that controls a polymer s behaviour is its molecular weight A polymer sample normally contains a range (distribution) chains of varying molecular weight PMMA PVC PS

3 Gel-permeation chromatography GPC (also known as size-exclusion chromatography, SEC) has long been used as a key tool for measuring molecular weight GPC separates macromolecules in solution It is based on the principle of separating the molecules according to their size in a chromatographic column After the column, the separated molecules can be analysed by one or more detectors Component A Component B Sample Solution

4 Conventional calibration-ri or UV detector measuring concentration Conventional calibration takes some standards of known molecular weight to create a calibration curve The unknown sample is then measured and its elution volume compared with the standards This has many limitations including, most-notably, that if the standard and sample polymers are different, the molecular weight will be only a relative estimate Polystyrene 400k as part of calibration curve PMMA is measured against this curve PMMA95k Peak RV - (ml) Mn - (kda) Mw - (kda) Mz - (kda) Mw / Mn IV - (dl/g) N/C Rh(w) - (nm) N/C Rg(w) - (nm) N/C

5 Multi-detector GPC GPC with static light scattering and intrinsic viscosity can measure absolute molecular weight and intrinsic viscosity, independent of elution time, structure and chemistry

6 Static Light Scattering Molecular Weight Measures the overall intensity of light scattered by a sample at known concentration, from which the molecular weight can be calculated at each data slice using the Zimm equation KC R 1 MWP 2A C 2 Where K = constant; c = concentration; R θ = Rayleigh ratio; Pθ = angular dependent term; A2 = 2 nd virial coefficient

7 How does intrinsic viscosity relate to structure? IV has the basic units dl/g Which of these two molecules with the same mass occupies the largest volume of space? IV is inversely proportional to molecular density IV 1/density We can look at structure in these terms: IV Volume/mass

8 Mark-Houwink plots The Mark-Houwink plot is used to compare the different structures of polymers It is often used to study conformation and branching

9 The multi-detection pyramid Branching Indirectly calculate: Hydrodynamic radius (Rh) Mark-Houwink parameters Directly calculate: Radius of gyration (Rg) Molecular weight (SLS) Intrinsic viscosity Concentration Composition Directly measure: Light scattering intensity δ Refractive Index UV absorbance δ viscosity

10 Adhesives and sealant applications Methacrylates, epoxies, polyurethanes, cellulose derivatives (methyl cellulose), silicones and many others Polymers, often with low molecular weights Few molecular weight standards Molecular weight, polydispersity and structure likely to affect: Adhesion strength Peel and shear strength Flow Toughness (extension and compression characteristics) Low molecular weights tend to be brittle Entanglement and toughness increase with MW

11 Epoxies & resins

12 Bisphenol A (MW = 228) Monomer for epoxy Pure sample has large RI signal with very low LS signal Mobile Phase: THF Columns: 2 X T2500 Flow Rate: 1 ml/min Temperature: 30C Injections: 100 µl of 3 mg/ml dn/dc: 0.195

13 Bisphenol A Diglycidyl Ether (MW = 340) Mobile Phase: THF Columns: 2 X T2500 Flow Rate: 1 ml/min Temperature: 30C Injections: 100 µl of 5 mg/ml dn/dc: 0.140

14 Epoxy oligomers With the right columns, oligomers can be resolved and their individual molecular weights measured IV RI LS

15 Methacrylates

16 PMMA processing Sample of PMMA run through 5 capillary rheology measurements simulating cycles of moulding and re-moulding (each line is overlay of 4 injections!) RI LALS IV Cycle 1-5

17 PMMA processing The changes in molecular weight distribution are significant The amount of high molecular weight material actually increases (each line is overlay of 4!) Cycle 1-5

18 PMMA processing Molecular weight increases with each processing cycle Increasing polydispersity will affect toughness could mean greater fragility or improved robustness means greater product variability and lower quality and value

19 PMMA processing After processing, there are clear differences in the structure of PMMA as well as the molecular weight (each line is overlay of 4 injections!) Suggests cross-linking Cycle 1-5

20 PMMA processing Rheology data Capillary rheometer results show some small differences in viscosity at low shear but similar viscosities at higher shear A bulk material measurement is not enough to see the underlying differences in this case

21 Polymer processing Different polymers respond differently to processing Understanding how polymer behaves during processing is key to a robust process PS PMMA

22 Polycaprolactone

23 Polycaprolactone (PCL) Biodegradable polyester Low melting point around 60 C Easily moulded simply by immersing in hot water Used in the manufacture of polyurethanes Plasticizer additive for PVC Controlled release/drug delivery polymer with slower degradation times than PLA Biodegradable implants

24 Polcaprolactone degradation SEC measurements of PCL Raw/virgin has the highest molecular weight Extruded at 80 C shows some degradation Extruded at 60 C in the presence of CO 2, which acts as a molecular lubricant allowing similar processing at lower temperatures protects sample from some of the degradation

25 Polcaprolactone degradation At first glance, the Mark- Houwink plots all look to overlay well At closer inspection there is a tiny but very repeatable difference in the plots Possible explanations Changes in branching levels due to extruding Changes in moisture content in absence of CO 2

26 Polycaprolactone rheology Small viscosity changes in response to extruding Virgin PCL has highest viscosity Decrease in viscosity following extrusion and concurrent decrease in molecular weigh Extrusion in presence of CO 2 can be performed at lower temperature and protects sample from some degradation PCL PCL extruded + CO 2 PCL extruded

27 Silicones

28 Silicones Three silicone samples Sample Injection Mw (Da) 10,335 10,164 10,983 11,407 11,656 11,761 11,096 10,875 Mn (Da) 4,375 4,141 5,981 6,857 6,696 6,924 5,630 2,695 IV (dl/g) Rh (nm)

29 Cellulose derivatives

30 Cellulose derivatives Derivatives of cellulose are commonly used in all sorts of pharmaceuticals Eye drops Moisturizing creams Raw cellulose can be derivatised in different ways: Hydroxyethyl cellulose Hydroxypropyl methyl cellulose Hydroxypropyl cellulose

31 Cellulose derivatives Hydroxybutylmethyl cellulose (HBMC) Hydroxypropylmethyl cellulose (HPMC) Carboxymethyl cellulose (CMC) Hydroxpyopyl cellulose (HPC) Methyl cellulose (MC)

32 Cellulose derivatives Different cellulose derivatives can be compared by number and on the Mark-Houwink plot These derivatives will have different effect on (e.g.) formulation viscosity This will depend on molecular weight, structure/branching and level of derivatisation Sample M n (g/ mol) M w (g/ mol) M z (g/ mol) [η] (dl/ g) R h (nm) HEC 62, , , HPC 45,600 69, , HPMC 98, , , HBMC 102, , , HPMC HBMC HPC HEC

33 Summary

34 Summary GPC is a key tool for separation and characterization of polymers Advanced/multi-detector GPC can be used to measure absolute molecular weight, as well as structural aspects such as branching and conformation, which will have significant effects on adhesive and sealant properties and performance The latest developments in GPC detectors allow measurements of lower molecular weight and lower dn/dc samples where light scattering sensitivity is a key concern

35 Thanks for listening Mark Pothecary PhD Americas Product Manager Malvern Instruments