PART 1 CHROMOGENIC MATERIALS

Thermochromic pigment materials and photochromic materials, electrochromic materials, halochromic materials, phosphorescent materials

Part 1a CHROMOGENIC MATERIALS-

Thermochromic, Photochromic & Electrochromic Materials, Halochromic Materials

• Chromogenic materials

  • Chromogenic materials change colour in response to an electrical, optical (e.g. light intensity) or thermal changes (i.e. temperature changes).

  • A chromogen is a coloured compound containing a chromophore - responsible for the observed colour.

    • A chromophore is a group of atoms within a larger molecule that are principally responsible for the absorption of visible light to give the compound its colour. The colour you observe is formed from the visible light that is transmitted or reflected by the chromogen.

• There are a number of different sub-categories of chromogenic materials e.g.

  • ELECTROCHROMIC MATERIALS

    • Electrochromic materials change their colour or opacity (how much light they let through) on the application of a potential difference ('voltage').

    • This effect is used in liquid crystal displays.

    • The term electroluminescence is also used to describe this effect.

    • Electroluminescent materials can produce brilliant colours if stimulated by an a.c. current.

    • Electrochromic materials can be used to light strips for decorating buildings and for industrial and public vehicles displays e.g. of safety notices.

  • THERMOCHROMIC MATERIALS

    • Thermochromic substances change in colour or become transparent depending on their temperature i.e. when the material is heated or cooled.

    • Thermochroism is the change in colour of a material with change in temperature.

    • Thermochromic materials change colour reversibly with changes in temperature i.e. rise or fall in temperature.

    • They can be designed to change colour at a specific transition temperature (or narrow range?), which can be varied by doping the material with other compounds.

    • Thermochromic materials are used to make paints, inks or are mixed to moulding or casting materials for different applications.

  • COLD

    transition temperature reached

    HOT

    • For example, you can paint or print a picture image of on something like a mug and then coat the image with a thin layer of a plain coloured  thermochromic pigment paint.
      • At room temperature (COLD) you see little if anything through the plain layer of thermochromic paint.
      • But, when you pour a hot drink into it, at a certain transition temperature e.g. 35^oC, the plain layer thermochromic pigment begins to become transparent.
      • Eventually the fully coloured image is then seen.
      • As the emptied mug cools down, the outer plain thermochromic paint returns to its plain non-transparent state and the picture disappears.
      • You can also paint ceramics with a pattern of different thermochromic paint pigments that change colour in such a way that the pattern itself changes with changes in room temperature.
      • A less artistic approach is to have a mug, cup or dish that changes to a colour that indicates the food or drink is too hot!
    • Designers think these are great materials for creative experiments e.g. shirts that change colour with temperature!
    • Simple 'strip' thermometers that change colour with rise in temperature, so you have a colour-temperature scale on a plastic strip you can place on your skin!
      • They are made using a mixture of pigments that become visible at different temperatures.
      • On the right is a picture of a simple household strip thermometer using a set of thermochromic pigments. It is reading 18^oC with a hint of 21^oC, they are not very precise but give quick and convenient indication of whether the room is warm or cool.
    • You can design electric kettles incorporating thermochromic pigments in the plastic body that change colour when the water boils.
    • Other domestic uses of thermochromic pigments include food wrapping or containers for food stored in refrigerators or freezers to indicate they are cool enough.
    • Mood rings use thermochromic pigments to change colour as the temperature of your finger changes.
    • Many baby products use thermochromic pigments as a safety feature to indicate if food/water is too hot to eat/bath a baby or young child.
      • Thermochromic pigments are used in baby baths, baby toys, baby feeding spoons to produce a colour that indicates things are too hot.
    • If you include thermochromic pigments in acrylic paints you can have painted walls or woodwork that changes colour at a temperature around room temperature e.g. you can have a blue paint below 25^oC and looking red above 25^oC!

  • PHOTOCHROMIC MATERIALS change colour in response to the intensity of light.

    • The term photochromic is applied to materials whose transmittance to light varies with the intensity of the incident light on it, so photochromic inks respond to changing light conditions.

    • Photochromic materials reversibly change colour with change in light intensity. They are often colourless in a subdued light but when sunlight or ultraviolet radiation is applied the molecular structure of the photochromic material changes and produces a different colour. When the uv/visible light source is removed the colour disappears (hence the term 'reversible').

    • Changes from one colour to another colour are possible by mixing photochromic colours with base colours which can be used in paints, inks etc. for many different applications.

    • Light sensitive sunglasses darken in response to increased intensity/brightness of sunlight and so reducing glare e.g. when driving a car or when skiing at high altitude when the snow reflects extra light into your eyes.

    • Light sensitive photochromic materials are used in optical memory devices.

    • Clothing can incorporate thermochromic inks that have patterns that change with change in light conditions.

    • For commercial use e.g. in consumer products (especially food) the photochromic inks must pass food container and packaging health and safety standards, and the producer must truthfully claim that they do not contain and potentially harmful or toxic substances.

Part 1b. HALOCHROMIC MATERIALS

• Halochromic materials that change colour if the ambient acidity changes i.e. in contact with an aqueous solution whose pH may change over time.

• One possible application is to use paints that can change colour in response to corrosion in the metal underneath them e.g. in the rusting of iron or steel the pH of the water in contact with the metal changes in pH.

Part 1c PHOSPHORESCENT MATERIALS

• Phosphorescence is the delayed and slow emission of light from a material that was exposed to some other radiation e.g. uv or visible light.

• Smart materials change their properties in response to an external stimulus.

• Phosphorescent smart materials

  • Phosphorescent smart materials are sometimes called 'afterglow' materials because they glow in the dark after being 'charged up' in the day.

  • Phosphorescent pigments absorb natural or artificial visible light energy and store it in their molecules.

    • This energy is slowly released over a period of time which maybe just a few seconds or several hours.

    • It is therefore a cheap and convenient 'rechargeable' light source for particular applications e.g.

    • Phosphorescent pigments have been used in emergency traffic signs, novelty decorations, evening lights in the garden (energised in daylight, toys and glow-in-the-dark watches.

    • The figures and hands of glow-in-the-night watches were painted with radioactive materials added to the pigment.

      • These radioactive paints will glow for years without needing repainting or charging in light (doesn't work like this anyway).

      • Unfortunately the dangers of radioactive materials were not fully recognised in the late 20th century and the human cell damaging potential was either ignored (not considered sufficiently dangerous) or not understood.

      • The girls who painted these watches and clocks had a habit of licking the paint brushes to moisten the tip hairs to make a finer point for accurate graphic work.

      • Tragically many of the girls died of cancer because they ingested small quantities of radioisotopes which directly irradiated skin cells causing cell DNA damage.

      • Fortunately research and development has produced safe phosphorescent smart materials that do the job completely safely.

• Note, but not sure if on any GCSE courses?

  • Fluorescence is the immediate emission of visible light from a substance that is irradiated by higher energy radiation such as uv radiation, X-rays or an electron beam.

    • Ultraviolet radiation is used to show up invisible anti-theft markers on valuable equipment or gain readmission to some pop concert or other event.

  • Chemiluminescence is light given out from chemical reactions in living organisms like glow-worms and fireflies and non-living things like light sticks.

WHERE NEXT?

INDEX of SMART MATERIALS PAGES

PART 1 CHROMOGENIC MATERIALS - Thermochromic, Photochromic & Electrochromic Materials

PART 2 SHAPE MEMORY ALLOYS e.g. Nitinol & Magnetic Shape Memory Alloys

PART 3 SHAPE MEMORY POLYMERS, pH and temperature sensitive-responsive polymers, self-healing materials

Part 4 LYCRA-SPANDEX

PART 5 High performance polymers like KEVLAR

PART 6 GORETEX, THINSULATE and TEFLON-PTFE

PART 7 PIEZOELECTRIC EFFECT (PIEZOELECTRIC MATERIALS) and PHOTOMECHANICAL MATERIALS

PART 8 CARBON FIBRES

See also INDEX NANOSCIENCE-NANOCHEMISTRY pages

and A general survey of materials - natural & synthetic, their properties & uses

[WEBSITE SEARCH BOX]

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*