Guest Diffusion in Polymer Materials

In the use of polymer materials, a significant number of research employ what is called “guest diffusion” in order to “dye” the polymer material a different color.  This process occurs based on polymer membrane selectivity due to the following factors:

  1. separation of differential solubility of guest molecules at the surface of the membrane (known as sorption)
  2. varying diffusion characteristics of different guest molecules or ions across the membrane
  3. desorption of molecules or ions on the other side of the membrane

For gas and liquids factors 1 and 2 are vital, while in ion transport all three factors are influential.  An example of the mechanism described in (1) is that hydrophilic molecules will dissolve in a hydrophilic surface faster than hydrophobic molecules.  However, in general, specific functional group determine solubility, attachment and entry into the membrane.  When diffusion occurs through a solid polymer, it is dependent on affinity and available space between chains.  In these cases, the molecular space can be influenced by whether the polymer is above or below the glass transition temperature.  When it is above Tg, greater chain movement is allowed, and therefore more free volume and permeability change for certain guest molecules.

The top label indicates the polymer membrane.  The left hand barrier is the sorption zone and the right hand barrier is the desorption zone.  The dots represent the molecules passing through the system.  The transmission of molecules across a membrane can be understood in terms of three processes--sorption, diffusion, and desorption--with separations accomplished if different molecules in a mixture respond differently to any of these three processes.

The top label indicates the polymer membrane. The left hand barrier is the sorption zone and the right hand barrier is the desorption zone. The dots represent the molecules passing through the system. The transmission of molecules across a membrane can be understood in terms of three processes–sorption, diffusion, and desorption–with separations accomplished if different molecules in a mixture respond differently to any of these three processes.

References:

1.  Allcock, H.  Introduction to Materials Chemistry. Hoboken, NJ:  Wiley, 2008.

Shape Memory Polymers Mixing

Recently in working on establishing the correct Tg of the SMP material, it was found that the previous paper used for the mixing formula had calculated the mixing ratios in error.  Therefore, I have cited the original paper here and present the newly calculated volume mix ratios to obtain the desired Tg value.  The shape memory polymer I am using is mixing three materials Epon 826, Jeffamine D230, and Neopentyl glycol diglycidyl ether (NGDE).  The proper mix ratios for these substances is given in the table below as used by Xie et. al. (1).

Screen Shot 2014-02-05 at 12.35.39 PM

In order to determine the volume mix ratios the molecular weights given in (2) and the density of each material given in (3)-(5) are used.  These values are listed below:

  1. Epon 826:  364.055 g/mole, 1.16 g/ml
  2. Jeffamine D230:  230 g/mole, 0.948 g/ml
  3. NGDE:  216 g/mole, 1.04 g/ml

Screen Shot 2014-02-06 at 3.29.35 PM

Note that the molecular weight of the Epon 826 is calculated from the chemical structure seen above.  Therefore, the new mix ratio for each formulation is given as:

NGDE1:  4.7 ml Epon 826/2.43 ml Jeffamine/1 ml NGDE

NGDE2:  3.14 ml Epon 826/2.43 ml Jeffamine/2.08 ml NGDE

NGDE3:  1.57 ml Epon 826/2.43 ml Jeffamine/3.12 ml NGDE

NGDE4:  0 ml Epon 826/2.43 ml Jeffamine/4.15 ml NGDE

The Tg for each of the mix ratios is determined by DSC  in Xie et. al. (1) whose results are given below:

Screen Shot 2014-03-06 at 8.29.16 AM

This paper suggests to heat the Epon 826 at 70 C in a oven to melt it for 15 minutes.  Then place the appropriate amounts of Jeffamine D230 and NGDE into the bottle and shake vigorously for 10 seconds.  Pour the mixture into the mold and cure at 100 C for 1.5 h and post cured at 130 C for 1 h.  Then cool and demold the samples.

The next step in my research is to try to modify this new procedure to properly incorporate nano fibers into the polymer matrix and see the change in mechanical properties.

References:

1.  Xie, T., Roussea, K., Facile tailoring of thermal transition temperatures of epoxy shape memory polymers, Polymers, 2009, 50, 1852-1856.

2.  http://www.faqs.org/patents/app/20080262188#b

3.  Huntsman Jeffamine D-230 Product Sheet

4.  http://www.sigmaaldrich.com/catalog/product/aldrich/338036?lang=en&region=US

5.  http://www.momentive.com/Products/TechnicalDataSheet.aspx?id=3937

Shape Memory Polymers: Heating Capacity Background

In order to affect change within a shape memory polymer, its properties above and below the glass transition temperature, Tg, must be know.  For my purposes, I wish to induce change through a heat source, more specifically electrical heat source.  Therefore, specific heat of the polymer needs to be known.  Heat capacity can be measured by adiabatic calorimetry (0-100 K), differential scanning calorimetry (DSC) (above 100 K), and some other techniques.  The heat capacity for any given polymer is a temperature-dependent quantity (1).   Reference 1, also gives a table containing the variation of the heat capacity of several example polymers with temperature.

In order to calculate the required power input to induce shape change, the First Law of Thermodynamics states:

ΔU = ΔQ – ΔW

meaning, the change in internal energy is equal to the heat added to the system minus the work done by the system.  Therefore, the calculate the power, the change in internal energy needs to be known.  (Power, in terms of electrical work at constant time, is given as W = VIΔt.)  For the change in internal energy, heat capacity can be assumed to be constant, giving the following:

ΔU=cΔT

where c is the heat capacity, and ΔT is the change in temperature.  In Reference 2, the authors are using a metal layer to induce a change in an SMP material.  Their approach to calculating the power required is by using standard values for heat capacity to calculate the change in internal energy.

Image

(a) Stiffness tunable composite embedded with Field’s metal strip and liquid-phase Galinstan heater. (b) When electrically activated, the composite softens and easily deforms. (c) Illustration of the composite composed of a (top) elastomer sealing layer, (middle) liquid-phase Joule heating element, and (bottom) thermally activated layer of Field’s metal or SMP. (d) Close-up of Galinstan heating element; ruler marks spaced 1 mm apart (2).

References:

1.  Mark, J. E., Ed.Physical Properties of Polymers, 2nd ed., 2007.

2.  Wanliang Shan et al 2013 Smart Mater. Struct. 22 085005